After braces, I guess, in principle, there's 3 scenarios:
1) In their new positions, all your teeth now "mesh together". As such, like with the rest of us, the teeth will tend to hold each other in place, so even if you don't wear your retainers, the teeth won't move. and you get to keep your perfect smile
2) In their new positions, all your teeth DO NOT "mesh together", thus when you don't wear your retainers, the teeth will want to move into some other position, and will tend to become stable in that new position. This is the scenario which mans you may need braces again.... and again!
3) (part-way between 1 & 2): In their new positions, all your teeth ALMOST "mesh together". If you don't wear retainers, your teeth will move a little, and your smile remains pretty good, just not quite as perfect as before. For people who started off with a very BAD smile, this situation is probably acceptable.
As a teen. my son had twin-blocks, to persuade his lower jaw to grow faster, and meet up with the top one: as no teeth actually moved, he never had retainers, and his teeth looked very acceptable. A few years later, as a young adult, his lower wisdom teeth arrived, and screwed thing up, but that is a 100% separate issue.
