They are also required, by law, to come to any address in the US, pick up a single letter, and get it to any other arbitrary address in the US for $0.49. Good luck getting FedEx or UPS to do it for 49 cents.
I’ve seen forever stamps marketed for years, but it’s been a long time since the price of a letter went up. So, if the price went up to 51 cents, your forever stamps would still work, but, honest question, what are the realistic chances of that happening?
[1] https://en.wikipedia.org/wiki/History_of_United_States_posta...