Instance diversity in Mastodon is that ~2M users or 70% of the network chooses one of 5 main servers. If your account is @foo@service.com and service.com goes in an undesirable direction (it shuts down, or is bought out by a billionaire, or turns on ads or paid subscription) you may be forced to switch to another platform like @foo@alt-service.com. This requires that the service.com continually upholds and honours your redirect, which is to say that your account name was never portable across different and incompatible services to begin with.

Network effects dictates that most of the time you will just stick to the same domain you signed up with, because you don't want to lose all your DMs and posts, and you don't want to start over again with a new name.