Unless the government plans to go back to the 80s, in which cryptography is heavily restricted from export, this doesn't accomplish anything.
And they can use it to converse only with themselves. Getting everyone else to stop using their favourite panopticon is the hard part.
All it takes is a usable interface, general availability, and a reason to change. If WhatsApp starts doing content moderation, of any form on the client, I am of the strong opinion that users will find this to be "creepy" and will immediately start shifting. As it is, all of my technical contacts have already left WhatsApp for Telegram. Since that group/audience has been the bellwether for all of the previous instant messaging client shifts over the last 2 decades, I imagine it's only time before "everyone else" also shifts off WhatsApp. I don't necessarily think it will be Telegram, but they definitely have the most momentum today when compared to, eg, Line, Zalo, Viber, Kik, et al.
That wouldn't accomplish anything now, anyway. All the current best crypto we have is out in the world, and plenty of crypto research happens outside the US. If the US revived the export restrictions, it wouldn't have much effect on the rest of the world.
It is a bizarre paradox where designing a secure system is a hard problem, where zero-days are commonly cheaper than brute forcing a 64-bit key, while cryptography is chronically lambasted.