Criminals get access to all sorts of illegal things, from drugs to weapons through stolen cars.

Are you telling me that you genuinely believe that they won't be able to download an open source, actually end-to-end encrypted app?

The stupid ones already use Telegram, which is not E2EE. There is no need to change anything for them. Those who are smart enough to choose a secure messenging app today will still be able to do it, even if that app is made illegal.