Even if signal puts in a backdoor to allow wiretapping (and criminals) access the next thing that happens is someone makes another application that doesn't have

Even if signal puts in a backdoor to allow wiretapping (and criminals) access the next thing that happens is someone makes another application that doesn't have it. Communication applications based on encryption aren't "hard" in the sense the maths is well established and a root of online business and a leaky system is worthless to businesses and many customers. Those that don't care will carry on with signal, everyone else will leave.