It is just... there. It does what I need it to do without standing in my way of communicating. It is also a standard that is not at the whim of a specific company shutting down a major service (that I am aware of) to continue communicating.
I struggled for a while after my mom got an iPhone for her to understand the App Store. I feel like she may not fully be there but is using an iPad now solely instead of a computer so its close...ish.
Maybe this is because I do use an iPhone and most people I communicate with are using iMessage that I just don't see the other issues mentioned here. But because I use iMessage, if for some reason apple were to shut down that service... I would just revert back to using SMS and communication will mostly just continue.
Finally... If I want to start talking to a new friend I meet at the bar. I just give them my number, no conversation about which app they use or OS ever needs to happen.
Edit: I know countries outside of the US are less dependent on SMS/iMessage. But it just feels like jumping through extra hoops for something that is built in.