I don’t use Whatsapp, I will never use it, and I refuse to hand my contact details over to anyone who does (because I don’t want Facebook to get my information).
However, others do use it. And the availability of software plays a pretty big factor in whether users will switch to it. Ask yourself this: do you want a proper Linux-based phone to become a viable mainstream option? Do you want other people to view this (or if someone else makes a different, but compatible device in the future - that) as a viable option aside from the current Apple/Android duopoly? Do you want people to see actual Linux and a device over which they actually have control as a possibility that they can use?
@guru: Consider that the main purpose of a phone is communication. This is, obviously, only possible if there are others to communicate with who use the same means of doing so. This unfortunate reason is why things like Facebook and Whatsapp are so big - people use it because people use it. It’s a self-reinforcing cycle.
I don’t expect that any kind of Whatsapp client will be made for this device in any reasonable timeframe, but if it ever does, you could see it acting as, for lack of a better term, a “gateway drug” to software and hardware that doesn’t sell you to someone else. A current Whatsapp user might not like what their particular hardware and communications software is doing, and want a new phone which can do things differently. But they’re not willing to suddenly drop their contacts on the old system.
If they have a device available which handles both their old communications method and something which doesn’t treat them like someone else’s property (for instance, if Matrix works out right), then they might be tempted to try out the new software. If it’s good, they spread it to their contacts, who might start moving away from the old system.
The best way to truly slay a monster like Facebook is to starve it. The only way you can “salt the earth” over its corpse to ensure that it never comes back is to ensure that people know exactly what it is that got done to them. But people aren’t going to immediately switch away to something else unless something completely and absolutely unignorable comes out. And they can only switch if they know about alternatives. To get people away from FB’s malignant tentacles, you need to a) make an alternative, b) make the alternative usable, c) make the alternative visible and d) make it clear that the alternative is superior (read: doesn’t stamp, file, index and profile them) while providing similar features.
Point c) here is what makes having a Whatsapp client… desirable (shudder). Someone who relies on that won’t even consider a Librem 5 or similar unless it’s available. But if it is, they might take a closer look at what else the phone offers. That’s the key. The initial hook.