For now I don't understand why this access rights segregation exist. Is it implemented for some security reasons? Or can it be needed by some use cases I can't imagine right now?
Knowing almost nothing about the reasons behind concrete PostgreSQL design I wonder why prevent a database owner from altering those tables of the database owned that are owned by other owners.
Here's what the official documentation on altering tables says regarding who can alter tables of a database:
You must own the table to use ALTER TABLE.
Of course I don't argue the design or the documentation. I'm just wondering why it can be needed to limit a database owner that way?