Android users can send SMS texts. I see it this way, here’s a person I want to tell that I’ll be there in 20 minutes. I send an SMS text to their Android phone. They reply, “OK”, letting me know they got the message… again from their Android phone back to my iPhone. This, right here, is a wonderful feat of technology! And, I wasn’t sending a video, I wasn’t sending a picture, just a text message. Some would say doing this would be called “suffering”. Some would say that it’s a “terrible” experience. However, in MY world, I just exchanged useful textual info with someone that will help us connect soon which is precisely what I wanted to do.
Actually, no matter what App someone may use to communicate, they can all also receive SMS messages. Sure, it doesn’t have all the frills of an app, but there hasn’t been a time when I’ve sent someone an SMS and they not be able to read or understand it. Outside the US, though, they’re likely to be charged to receive that SMS AND, charged to send a reply back. If someone feels their messages are worth another person paying a few dollars a day (and the recipient feels the same), then they can use SMS indefinitely. Most would probably rather not have their phone bill balloon because of texts going back and forth about the plot of a recently seen movie and its implications to the characters, though. It’s not that they don’t want to chat with the person on the other end, it’s just feels better when the volume of communication doesn’t come with an added expense.
Yes BUT there’s no two-way street for messaging. One either uses WhatsApp or one doesn’t use WhatsApp, there’s no compromise. If I want to communicate with someone else more than they want to communicate with me, I use their app to make it easy. This is a self-serving decision for me because I value their conversation and engagement more than any question of technology. I believe this is what most people are doing and why WhatsApp has the marketshare it has. It’s not that everyone got together and said, “Alright, we’re using WhatsApp”. When someone got a phone, they checked to see what others were using and just accepted it, likely without much emotion at all. It was a one way street, “We’re using this and if you want to chat with us using your phone, you’ll use this”.
You’re asking that something which isn’t, and has never been, a part of “basic smartphone functionality” become a part of basic smartphone functionality. And, the reason why it’s not basic smartphone functionality (and unlikely to become a part of “basic smartphone functionality”) is because each company has built their own messaging functions separate from the others. And, when dealing with encryption, the fewer folks that hold the master keys, the better so interoperability isn’t likely in the cards.
I can tell you RIGHT now, that if I knew a friend or friend of a friend that had a penchant for randomly messaging folks on their messaging app and offering them flights to the vacation they’ll be going on next week, on a whim, you’d BEST bet that I’m installing and turning on notifications for WHATEVER app is their choice.

If there’s no one on WhatsApp or FB messenger that you want to communicate with, why would you install them? If there was, and that’s the ONLY thing they use, (and remember YOU want to communicate with THEM) why wouldn’t you?