Oh they're (probably) lazy like everybody else, with the difference being that they have something to hide that will potentially put them in jail.
I'm not too worried about your average "small scale" criminal suddenly becoming a criminal mastermind, but organized crime will certainly adopt safer ways of communicating, and those are the people you want to catch with electronic surveillance.
The small scale criminals usually leaves plenty of other clues that will allow the police to capture them.
These types of organizations learn to build their own submarines to transport drugs all over the world undetected. I think they can manage a fork of Signal/Session with any backdoors removed.
With sufficient motivation (money is a great motivator), everything is possible, especially if not doing X will remove said money, and/or put you in jail.
It's not even like they need to fork Signal/Session, they could get by with GPG encrypting a gist and uploading that, sharing the link in signal or wherever.
As I initially wrote, weakening encryption only harms law abiding citizens, as everybody criminal probably faces much worse charges than breaking encryption laws.
And maybe they are even more lazy than average people because that's why they became criminals in the first place.