What would the logistics of this be? Would the government need to store all master keys to be able to decrypt an old message? How would you know you're using the right key to decrypt a message? What happens if all the old keys leak?

What about foreign communications? You can't compel foreign actors to encrypt with your algorithm. What if I'm storing foreign data which is encrypted with illegal algorithms, is that going to be illegal? If so, then goodbye hosting services in the US. If not, how are you going to differentiate between foreign data and local data?

What about the transition period? What do you do with legacy encryption? What about people who haven't received the newly updated government-sanctioned encryption yet? What about old devices that can't run your encryption algorithm, closed systems, etc?

I don't think it's as incredibly simple as you put it.