With the Old Way:
Designer: does N work to create a design and has X% chance of winning $2500. They focus on what they love (i.e. design) and can get a nice quick payout if they're good. This is even better than a lottery, because it has the same excitement, but talent is also a factor which is very motivating. The marketing is done for them. Competing head to head is also kind of fun.
Shopper: look at the top / winning shirts and buy one if they like it. Paradox of Choice is not a problem.
With the New Way:
Designer: needs to setup a shop, do N work to create a design, then M work to market the shirt, and needs to sell nearly 400 t-shirts to get to that payout. For the winners to achieve the same outcome, it's a lot more work. For most designers, I think it's safe to assume that the marketing part is nowhere near as fun as the design part. The excitement factor is totally gone.
Shopper: has to wade through tons of shirts to figure out which one they like. Paradox of Choice is signifcant.