If providing was both completely legal (as it is in some countries) but also considered respectable enough for a conversation at the average Thanksgiving dinner (does not exist in any country or culture afaik), do you think the provider:hobbyist ratio would change in any way, and how so?

On the provider side, I am 100% sure there would be many more providers, which ultimately in my opinion would lead to lower rates, or at least more providers would be open to many different services.

I wonder if it would even exist anymore.

used car salesmen or an ambulance chasing shyster lawyer would be less frequently used.  
 ( with "apologies" to any used car salesmen and less then ethical lawyers)

there are some lawyers who aren't ethical?  And what exactly is wrong with being a used car salesman?  How is that different from all of the used pussy saleswomen we have here?  I'm willing to bet they both misrepresent the actual mileage.  

what the hell is "respectable" Sex?
Do I have to leave out the filthy stories and slurpy blow job?

Even in countries where it's legal and there are a whole shitload of such places. I've probably paid for sex legally in over 20 countries, but it's still not considered a "respectable" way to make a living by most.

Apology accepted.....

Just to answer the hypothetical, respectability and legality would both increase the supply, but also the demand.  It is hard to say how that would affect the ratio.

The whore of Babylon:

Lower prices because woman who enjoy would appear out of nowhere and do because they enjoy.  Lower price because no risk of arrest, no need to hide.  

the Wife would rather have you pay $100 (more for manicure & pedicure)
for her to sit and have someone pamper her.

My psychiatrist asked me if I thought sex was dirty. I said it is if you're doing it right.

666 to you, too, Fishbro!

There would be a LOT more providers, especially since this job doesn't require any skills to enter. There would also be a lot of unhappy clientele.

Jealousy is being ripped wide open  
with intense study, and lots of authors,
 on EMOTIONS.

I was in the industry in Australia, I was a bouncer in a legal brothel. Similar economic and cultural demographics as here. It's legal, but there is always going to be judeo/christian moralists who are closed minded and prejudicial to sex workers and their clients.

I have to ask if you really think you can reason someone out of jealousy?  Now try to do that to the 4 billion people living on earth.  We have to pick our battles.