The idea is you can subscribe and interact with any instances, no matter what instance you came from
Sure, most will jump the bandwagon into lemmy.world.. at least for the near future. I can say that with confidence because when you search "how to join Lemmy", most guide will point you to Lemmy.world instead of.. lets say.. coughLemmynsfwcough
Over time, some will eventually move to other instance (mostly just the account, because they want 'cool username' for themselves). Sooner or later, things will balance themselves out.
Maybe you can even start by deploying your own instance, for no other reason than claiming your own 'cool username'.
You made an account on lemmy.world. You can't login to mastodon or any other lemmy instances, but you can interact with them.
You can access communities in any other lemmy instances from lemmy.world. So, instead of directly acessing otherlemmy/c/something, you access it from lemmy.world/c/something@otherlemmy
For mastodon, if you want to use mastodon you'll need to make a separate mastodon account. Meanwhile, you can see your lemmy.world account posts from mastodon. Just login to Mastodon and go t o mastodon.world/@[email protected]