It's not about being reachable in a 1-1 chat. It's about being excluded from group chats which are very important for keeping up the group and friendship in general. For example, if I deleted FB Messenger right now I would still get invited to the occasional event but there would be way fewer "yo, I'm at X, is anyone free to hang out?" type messages that I will see, or invitations from acquaintances in big group chats, or the current shitpost of the week that will become part of everyone's lexicon for the next 2 months other than yours, etc. For a lot of people (including me) such communication is a majority of their overall communication with their friends and breaking them does break a big part of their life.