Don't have the answer but it's one of my absolute dream that fixed braces could be used as a retainer after a treatment, temporary or permanent, to make the treatment last more and more.

Thanks, so if they said you need to wear 24/7 retainer after braces for two months then leaving the braces on for two months instead accomplishes the same thing?

Thanks!

It has always been my understanding that something similar to this is what orthodontists used to do, before.it became fashionable to get braces off the moment teeth were straight.

Not for a major length of time, but on a sort of "your teeth are straight, so we'll remove your braces next time - 6 weeks later or whenever' so they had a period of being held in place without much pressure from the braces - especially knowing that few people are 100% diligent with retainer wear.

Certainly I recall a number of my friends being told something along those lines.

Agreed. My wife had a period of no treatment during her braces due to various factors (moving house, ortho retiring, covid) for 5 years or so

She is meticulous with her oral hygiene and had no issues with cavities or decay. Her braces at the time were self ligating with steel archwires and she had wire lacing on the front 6, upper and lower so there was no active pressure on from NiTi wires or elastomerics

However, over time she did have some spaces open up, particularly her (almost closed) extraction gaps, which opened up to 5mm or larger. They weren't protected by wire lacing however

My guess is that the primary consideration would be oral hygiene.  If you are scrupulous about keeping everything clean, my guess would be that fixed appliances might even be preferable as Essix retainers wouldn't allow circulation of saliva.  That might even be true on the lingual surfaces with Hawley retainers.  You'll probably have to earn the right to ask for the extension.