Anyone blindly believing every random allegation is also a fool, especially when the app in question has been thoroughly reverse engineered and you can freely check for yourself that it's using the same protocol as Signal for encryption
I don't have any proof that Meta stores WhatsApp messages but I feel it in my bones that at the very least tried to do so. And if ever that comes to light, precisely nobody will be surprised.
The amount of ambient cynicism on the internet basically makes this a meaningless statement. You could plausibly make the same claim for tons of other conspiracy theories, eg. JFK was assassinated by the CIA/FBI, Bush did 9/11, covid was intentionally engineered by the chinese/US government, etc.
On the other hand, Occam's razor can barely keep up with the mental gymnastics required to paint Bush (or even Cheney) as the mastermind behind 9/11.
The evidence is pretty clear that Facebook wants to do everything they legally can to track and monitor people, and they're perfectly okay crossing the line and going to court to find the boundaries.
Using a company like that for encrypted messaging seems like an unnecessary risk. Maybe they're not decrypting it, but they're undoubtedly tracking everything else about the conversation because that's what they do.