Back in the 90s I started playing with PGP. I created a private/public keypair and I started signing emails and newsgroup messages that I wrote. Of course, no one was trying to impersonate me, so this was a complete waste of time, but it was good clean fun.
What I really wanted to do, however, was send actual encrypted messages. This required someone else get involved in my little fantasy. I published my public key everywhere, but just like no one was interested in impersonating me, no one was interested in sending me encrypted messages either. There was, however, a girl that was interested in me for other reasons, and with a bit of fast talking, I was able to set up her computer so that she could send me actual encrypted messages.
It was a bit of a pain, and at least on two occasions I had to go over to her apartment and help her unlock her secret key so that she could actually read what I had sent her, but it did give me a chance to actually use military grade encryption! If you squinted hard enough this even seemed like something that normal people might do.
Except that they never did. Not even criminals ever switched to using encryption. More than anything this shows that criminals are idiots. I mean, honestly, it is not that hard. You really only need to exchange public keys, and you can do that in public.
What Signal (and other programs like it) have done, was remove that particular barrier to entry. When I use Signal to talk to my friends I don't have to worry about what their public key is. Signal handles that part. Nowadays that's not even just Signal. Google messages handles it for me for basically everyone using Android. And yes, because they handle it there are lots of ways that this can be abused. However, it is mostly good enough, and it is a way better default than just sending plain text. More importantly, it is easy enough to do that for the first time in my life people are using end to end encrypted communication by default.
Law enforcement can try and put that particular genie back in the bottle, but I don't think that is likely to work. It is simply too easy to pick a service that does mostly the right thing. Honestly, I think the real thing holding people up from using encryption before is that the cryptography nerds wanted webs of trust and other ridiculous strategies when something as simple as letting Signal handle matching public keys was good enough. With my hard core nerd friends I did verify their public key by hand (the first time I set it up). When they got a new phone I realized that ain't nobody got time for that. It still works way better than anything else I have ever used.