I still can't understand what's wrong with this. I believe we have normalized being constantly reachable and available way too much, and "through mandatory text replies" is already way too much. Calls take this one step further: "I am demanding to have your undivided attention, right now, for as long as it takes, I don't care what you're doing". I just think that's rude.

Actually, even with my partner we have a "Scheduling calls is vastly preferable to random calls" and I am 100% okay with this. If I am doing something else and it's not urgent, I'll get to you later. Let me get my work done and wait until my next break, or let me actually enjoy my friends' company IRL for a few hours, then I get back to you to chat. Why do I have to be available, at your disposal, immediately and giving you my undivided attention anytime? I'm not a chatbot, I'm a human being with a full and interesting life.

I believe not doing this is only doable if you have few friends. If you have plenty of friends + a full social life, you really have to manage your conversations and find various time windows throughout your day to keep up with multiple texting threads and that is time consuming as it is - before I established my own boundaries, it would seep in all areas of my life and I would get absolutely nothing done at days because it was too dispersive.