As I said elsewhere: this would still have been the case if Google hadn't used XMPP in the first place. All those people you have lost when Google defederated either wouldn't have been on XMPP at all or care enough about privacy and open source to have both. It's not like you have lost anyone who had been there before Google started using XMPP. Same with Threads. Every single user who is on the fediverse today will still be here if they defederate (unless they leave for other, unrelated reasons of course). Considering our existing userbase's interests, I don't see many people giving up their existing accounts in favor of a Threads account.