This is assuming the marketplace works perfectly... Which is an incorrect assumption. Reality is that the marketplace is highly controlled by algorithms. New platforms will struggle to get exposure... No exposure, no credibility, no word of mouth, no users, catch 22... You think the big players will allow small SaaS projects to gain traction on their platforms? Have you seen how centralized the Internet is these days? Have you seen how afraid people are of betting on no-name platforms? If they choose the wrong no-name platforms and tools, they will lose their (increasingly precious) jobs. As the saying goes "Nobody lost their job for choosing IBM." As for B2C; it's dead, consumers don't have money and will have less of it in the future; the mass-market game is over.

My bet is if there were a lot of great apps being built, even excellent quality, nobody would even hear about them. The big players would copy them before anyone even found out about them.

IMO, the market is not even a playing field anymore, this is why everyone is getting into politics now, though politics is also somewhat monopolized, there is still more potential for success because there is such an abundance of dissatisfied people willing to look outside of mainstream channels. It's much easier to sell political ideologies than to sell products.