But again: how does the account system switch matter for any of that? Like, what is the thing that's suddenly possible for them to do when you log in using a Go

But again: how does the account system switch matter for any of that? Like, what is the thing that's suddenly possible for them to do when you log in using a Google account rather than a Fitbit one? If they're willing to maliciously (and at ludicrous risk for low payoff) do some kind of cross-account sharing of health data for unrelated purposes, why would it be any harder to associate a Google account with a matching Fitbit account, than to associate two Google accounts? You seem to be making an argument, in vague generalities, for why they shouldn't have been allowed to buy Fitbit. But that deal was approved by the regulators across the world, after concessions, so there is no point in relitigating that. Is there a concrete argument for why they should require indefinite support for a legacy account system? There is a pretty obvious reason for why people cannot article why they feel negative about this... Because they are feeling negative about something else entirely, and projecting. But this particular change is be strictly an improvement: it's going to be much better to argue against the things that actually bother you, not use this as a proxy.