Matrix however is a great choice imo.
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.
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.
Forcing your users to first install one more messaging app is going to cause people abandoning you immediately or slowly churn, which can make or break a product.
On top of providing a better developer experience compared to Meta's ultra-locked and limited APIs, they aren't subject to the whims of a giant faceless company that can kill your product for no apparent reason with no chances of appeal.
Especially if you're doing these projects for folks in the developing world. Let's not lock them in proprietary American spyware like the whole West has already done :) from a user's perspective, if things are done properly, it'll just be a matter of installing another app.
And btw using a Matrix server with a WhatsApp bridge could also be a temporary solution to bypass the ban. But I haven't tested it with business accounts.