The tube sites are a huge issue. But so is the fact that anyone with a webcam can make a site and do "amature" stuff pretty easy. So the "talent" pool is decreasing.
Then the star power kind of fades because the big companies are losing some money at it because of the competition. So they don't throw the money trying to market and promote stars like back in the day with Vivid video. Remember all the girls wanted to be considered a "vivid girl". I mean there was a thread about a girl who filmed herself masterbating in a library and now she is considered a "headliner" at an adult expo. Also some of the companies treated their "stars" like shit so they all moved on and started their own production companies and what not. So they had to be self promoters. Which is always a tougher row to hoe so to speak. Then what I really think what the down slide has become is that the industry has changed so much. Remember in the 90's there wasn't every woman doing anal, dp, gape, gangbangs, BBC, etc. I mean now the first filming they are asking the women to do some if not all of these. I have a friend who recently got into porn. She got paid $500 per scene and her first scene was a three way with anal. Her next scene was a BBC with anal. She didn't do any other scenes because she was all used up (in her own words). So these production companies are burning up the women.... IMO. Now again this is a smaller production website based company. But those are a dime a dozen these days. Where those used to be how women got a foot in the door but now are getting burned out because of the "extreme" stuff they are asking them to do at an "entry level" type production company. But yeah..... I know what you mean it is hard to find some of your ATF's out there because they retired so quicklySo I'm back in the game after a prolonged absence, and after noticing that "if you wants to play, you got's to pay!," I've seriously been considering seeing some of my ATF PSs.
Problem is, when I searched the review archives here on TER, almost ALL of my ATFs are outta the biz!
Then when I check out the Exottica thread posted here and clicked on their site, I found that I didn't recognize over HALF the girls on there?!
WTF...do I have to go ahead and trade in my ATFs and start the search for some NEW ones from scratch? I tell ya, it's enough to make a grown man cry! 
Porn (and its affiliated businesses) has never given EXPERIENCE the respect it deserves. Sure, it's a biz that ANYONE can get in (and does), but how many are TRULY good at it? How often does an "Ava Devine" come around? (...and she's retired, of course!).
You'd think with the ever diminishing stigma attached to the biz, some pros would stick around for awhile. But it seems like it's 1, 2 years and you're out, and the biz is onto the next NEW thing. Trust me, as far as this biz is concerned, youth is OVERRATED and experience (and GOOD service) is UNDERAPPRECIATED. Just my two cents. Please feel free to chime in. thx!
The problem with porn is that there is little money to be made now as a pornstar. Largely because of tubesites and piracy. I know what you mean though about girls retiring. All my favorites girls from the mid 2000s are retired. I feel blessed though getting the chance to meet my favorite pornstar when I did. I hardly follow porn at the moment and I didn't recognize the newer 'stars' either.
Just think of all the fun you can have searching for new ATFs. 
Seriously though, I think the turnover is good in the long run because I like variety. I like meeting new ladies, and enjoying their company. Yes, I do repeat when I find one I like well enough, but I have plenty of fun making new memories and having new experiences. While youth may be overrated, and I agree that there's something to be said for experience, I like meeting the ladies while they are still fresh, unjaded, and before they've met all the creeps and losers that are out there and have become cynical. I also like being in the teacher role and teaching the ladies some new things, or giving them new experiences.
BTW, a woman's time in the porn industry has ALWAYS been limited to a few years. That's not new. I think it's gone from an average span of about 5-6 years to 3-4 years which isn't all that much of a difference. The good and popular women still do 100-200 films/scenes (or more) while the bad ones wipe out after a few. The average still seems to be in the 50-75 films/scene range. That's discounting cam girls and cam work in general since the number of "performances" would be impossible to count.
I like "variety" as much as the next guy, Mr. French. I just find that as I get older, I appreciate that in this field, as in any other endeavor in life, there are those who are GREAT at it and those who are just mediocre. That's why you have certain athletes earning more than others: they possess certain skills and abilities that others just don't, and those players, your Kobes, your Jordans, your Cal Ripkens, tend to stick around for the long haul as well.
I'm just saying people on both sides don't appreciate QUALITY and SKILLS the way they do in other professions. I'd rather be with a seasoned "old" pro who knows what she doing, than some eager fresh off the bus kid who thinks flashing her boobs is the HIGH point of erotica.
The tube sites are a huge issue. But so is the fact that anyone with a webcam can make a site and do "amature" stuff pretty easy. So the "talent" pool is decreasing.
Then the star power kind of fades because the big companies are losing some money at it because of the competition. So they don't throw the money trying to market and promote stars like back in the day with Vivid video. Remember all the girls wanted to be considered a "vivid girl". I mean there was a thread about a girl who filmed herself masterbating in a library and now she is considered a "headliner" at an adult expo.
Also some of the companies treated their "stars" like shit so they all moved on and started their own production companies and what not. So they had to be self promoters. Which is always a tougher row to hoe so to speak.
Then what I really think what the down slide has become is that the industry has changed so much. Remember in the 90's there wasn't every woman doing anal, dp, gape, gangbangs, BBC, etc. I mean now the first filming they are asking the women to do some if not all of these. I have a friend who recently got into porn. She got paid $500 per scene and her first scene was a three way with anal. Her next scene was a BBC with anal. She didn't do any other scenes because she was all used up (in her own words). So these production companies are burning up the women.... IMO. Now again this is a smaller production website based company. But those are a dime a dozen these days. Where those used to be how women got a foot in the door but now are getting burned out because of the "extreme" stuff they are asking them to do at an "entry level" type production company.
But yeah..... I know what you mean it is hard to find some of your ATF's out there because they retired so quickly
I agree with what everyone is saying. Gotta remember a couple of things.
1. The young ones only get into it to get money for college.
2. Their family finds out.
3. They start a family of their own.
4. They get a job in the "real world"
THe main thing is that most get married. Most of the time when you meet a PS, they don't tell you about their personal life. I recall seeing a UTR one time and she was telling me she was definitely single with no attachments. Then the next week, i found out that she became engaged and moved on with her life.