The disposable part is exactly for that. However it is not strictly necessary imo, just “something more” that doesn’t cost something.
Regarding qubes providing the best protection against actively hostile networks: Beause you can easily deploy as many netvms/firewalls as you like with as many software diversity as you can manage.
You can have persistent configuration nevertheless! Just drop them in the dispVM-Template! But if you add a new network you need to fire it up and add it there, so it is mildly more cumbersome then.
Just one idea that came to my mind at this moment:
Maybe it is even possible to automatically rotate dispVM netvms every few second that are on wastly different systems like arch, fedora, debian, ubuntu, nixOS, all them BSDs to completely confuse an attacker trying to fingerprint and attack. One can have a lot of fun.