What would prevent Lemmy from becoming another Reddit

You'd need to convince every one hosting a Lemmy instance (and all possible future ones) to sell their platform for money or blackmail them into submission...

With Reddit, they only needed to convince Spez which is like convincing an alcoholic to have a free shot of his fav poison

If Lemmy, or any other fediverse social network, ever got that big, you would be guaranteed there would be plenty of splinters of de-federation in the network. There would be small networks totally isolated from the mainstream cluster, and others that only federate parts of it. You could choose to hang out in some counter-cultural bubble, or choose a curated connection to the biggest networks, if you didn't want to engage in them fully. The trick would be finding "your people," but the tech works.

reddit and all commercial social media make more money when they have more users, so they attract a lot of users with ragebait and clickbait.

we don't profit if we have more users, so we don't have an incentive to send clickbait and ragebait, and the absence of these toxins alone will probably drive up the quality a bit.

It’s not moderated the same way as Reddit