Switching is super hard when your primary platform just fallback to SMS.
If there's interop between the chat applications - and be honest, do any of them have a unique selling point that makes them significantly better to the average user? - then that restriction is lifted.
It improves competition, which is healthy in a free economy (something corporate liberals really push for), forcing the chat apps to innovate instead of benefit off of critical mass / first mover advantage.
Sure, but "the old one is no longer available" is absolutely going to get everyone to switch in the approximately 60 seconds it takes to go from the notification to having a new account.
I mean, Google basically does this every time they change the default Android messaging app (though in fairness they do get a huge advantage of already knowing all your contacts): https://arstechnica.com/gadgets/2021/08/a-decade-and-a-half-...