I don't know whether this also applies to ActivityPub, but with Matrix it's similar to e-mail. If you use an identifier supplied by your instance, you run into

I don't know whether this also applies to ActivityPub, but with Matrix it's similar to e-mail. If you use an identifier supplied by your instance, you run into the issue of having to tell people your new address. If you use your own domain as an identifier, you can simply redirect it to another instance if you switch.