I don't care about prize support but:
If I am paying a substantial fee to play, then I expect there to be support. Renting space is expensive and it all compounds on itself
I won't endure the expense of substantial travel (requiring hotel and likely airfare) for a small event or for no added incentive
The added incentive of title was always the big draw for me at eternal weekend. Winning the title of world champion is worth so much more than credit, art, or cards
I would say that Old School might have some influence on turnout, but I think calling it the reason is not accurate. I think that many people are just not as interested because the meta is not interesting anymore (for some - like me). I expect that will not always be true. These things shift all the time. In the interim, I'd rather spend my time on a format that I enjoy more at the moment.
In a sense, I may have gotten out of Vintage had it not been for Old School. I've certainly been tempted to sell out with the crazy price spikes of the last few years but my love for Old School has kept my cards in my hands. I know part of that spike is due to Old School in the first place, but if I hadn't picked it up myself, I'd be far more tempted to cash out.