Being opt-in for the WhatsApp user reduces the severity of that problem a lot, though certainly not to zero for those who do opt in.
TBH it also reduces the usefulness of the features a lot. If I first have to contact people on another channel to tell them to enable this feature because I want to message them, well, I guess most people wont just do it.
I'm more worried this happens because meta wants to access other networks to train their algorithms for free - just like with fediverse integration on threads.
Of course, this is still only an unfounded guess but I can't believe they're doing this selflessly, out of the goodness of their hearts.