This reminds me of a thread a few days ago where I pointed out a similar thing you’re saying here. I think you make a strong case that, in the business context, this tie in is mandatory.
It’s all well and good taking a principled stance on not using FB/WhatsApp/etc. While you’re doing that, your competitors are selling out, using them, and getting to your customers more easily than you are. They might get cut off but that’s tomorrow’s problem, while your problem is here and now.
Facebook owned the users. Similarly, WhatsApp owns the users.
The choice between:
1. Tying yourself to a platform that could boot you at any time, while also reaching all the users they already have
2. Bootstrapping something from scratch with 0 users, when all the users you want are already using said platform and have little incentive to leave
Choice 1 is often the only viable option, particularly if you're financially dependent on the results.
I also think we repeatedly see, even on HN, that "it'll just be a matter of installing another app" is actually a huge mental hurdle for many people - especially those who aren't technologically savvy enough to know how to use something that isn't Facebook/WhatsApp for communicating with people.