You're right, long-term identity keys are bad. Long-term identity keys are not a concept mandated by PGP, they are a result of how people use PGP or how PGP is implemented in a third party app.

No part of PGP requires you to use a key more than once. This phenomenon is a result of a consensus of people deciding on a terrible operations strategy over a long period of time.

Edit: this comes to mind https://gist.github.com/grugq/03167bed45e774551155