You can say what you believe the “philosophy of the fediverse/foss” is until you’re blue in the face, but there’s literally nothing in place to enforce what you’re saying except for the users. Someone could start an ad supported instance tomorrow, and if people use it, then they use it. The Lemmy devs can’t compel them to remove ads, that’s not part of the license (afaik, I’d be glad to be proven wrong), nor would they compel instances to defederate from them. That is the opposite of the fediverse philosophy.
I’m getting down voted for acknowledging reality, but it is my firm belief that the moment we ignore this reality is the moment someone steps in and exploits it. In order to maintain the fediverse as we want it, it takes a user base that prioritizes choosing instances that don’t serve ads over ones that do, and not a user base that thinks ad supported instances aren’t possible. Meta’s Threads was an obvious attempt, and I’m glad that most instances unanimously agreed they were antithetical to Lemmy, but it won’t always be that easy.
And I 100% agree that migration of communities and accounts between instances should be top of the list of Lemmy features. Without that, then yes, as you say, any communities/accounts on that instance are lost, or worse, keep users on an instance that hurts the community. But even still, unlike reddit, that would only amount to a small subset of communities/users, and not 100% of them.