Changing usernames when you change instances is also a very significant problem. Are there proposals for addressing that?
That is a point though: moderators change over time and a moderation team you like one year may be grossly different 5-years from now. To "leave" to a new instance means to give up your old identity entirely.
You lose followers when you change instances but you can set a flag on your old account that points to the new one.
I think you just summed up precisely what makes federated instances the best solution we currently have available. As your comment shows, there is no perfect balance for everyone--no matter what kind of moderation we put in place, there will always be that group who will be unhappy.
With federation, if you think you'll be unable or unwilling to follow an instance's code-of-conduct, you simply hop onto a different instance which better fits your personal expectations; or if you are very very particular, you just toss up your own instance et voilà, you're talking to those on the Mastodon network with your own ideal of a perfectly balanced system.