Just hope one wins and everyone posts there

That's one way.

Fragmentation exists even on centralized systems, though. (How many Dungeons & Dragons subreddits are there? How many are there about cats?) So it's probably best to temper expectations of one forum to rule them all, regardless of what platform we're on.

or is there a technical solution?

We may start seeing fediverse clients that can group related communities together and show them as a single feed, like multireddits.