Earlier quoted context omitted.
This comes up everything the EU/Europe and Apple comes up: iMessage is hugely popular, it is just not the most popular (or even third or second) messaging platform. Also the largest is Facebook Messenge, and in the top five is another Meta product, WhatsApp. So it's not like it's all Signal, Viper or Telegram, it's mostly platforms from another US tech giant.
Switching is easy
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.