Well if instances keep defederating each other, it does matter. Plus there's the question of stability. Sure, you could make your account on some tiny niche instance, but what if the guy running it decides he's had enough and terminates it? What happens to your account, your post history?

Sure, but it'll be the same story all over again. The big platform will be ruined and the alternative option will be smaller and therefore not as good (since user base directly contributes to quality when it comes to community-based platforms; it's the users who post all the content, so fewer users = less content).

Eh, don't be so sure.

Email is often drawn as something similar to the fediverse. ... but if you've ever tried to run a small Mailserver, you'll quickly find that "the big corps" have created a walled garden that'll keep the "small fish" out.

It's all based on what the big players view as your "reputation". This is based on proprietary metrics (usually how many emails you send), but your reputation will determine if the email is delivered or not.

You can find more information here.

... but the point is that one big corps consolidate and reach the size (in terms of traffic/content) like Hotmail, Gmail, yahoo, etc - they will not hesitate to squeeze out the smaller fediverse fish to force them into paying to use the bigger pond.

Sadly ... this is just business as usual.