It's interesting how differently the data science world has embraced this model vs. designers, of which a vocal set seem to scoff at any mention of 99designs. (I am currently competing in a Kaggle competition and the competitive factor is what makes it fun)
That said, I doubt more than a handful of people could make a living off competition winnings alone.
Kaggle is pivoting, and they don't intend "making a living on Kaggle" to involve contest winnings. Instead, they are offering a platform for prospective employers can use to contract to highly ranked Kaggle members.
Good designers scoff at 99Designs because the expected payout is absurdly low for the amount of skill and effort usually required to win a "competition". The price structure undermines the pricing of struggling freelancers trying to break out in the low- to mid-tier end of the market. Bean-counting execs who don't care about design see no reason to pay $X000 or $XX000 to a pro designer with legitimate expenses when they can get several useable designs (I won't say "good") on spec for only $Y00. Nevermind that many of those crowdsourced designs come from kids and people overseas using pirated versions of Photoshop.
Besides the economic damage wrought by 99Designs, a lot of designers have gotten a sour taste in their mouth from the blatant copying/IP theft that often wins. If I come up with great idea A, and you do a nearly identical rendering of my idea but in what happens to be the buyer's favorite color.... who do you think will be selected as winner? Unfair outcomes are pretty common, especially because the people doing the choosing often wouldn't know a good design if it bit them in the face.
That said, I doubt more than a handful of people could make a living off competition winnings alone.