ok, WhatsApp. First, ugh, Facebook, please stop this shit of messing with the orgs you buy. Second, however, WhatsApp remains *good enough* for most people. It has a wide install base and it's easy to use. Moving from it is a non-trivial hassle for not-very-techy folk.
If your dad wants to continue using WhatsApp for family chat, please don't give him a hard time. The content remains encrypted. Yes, I know the metadata is a shitshow for privacy, but for most ordinary folk, that is a) abstract and b) not a massive concern.
Most people - including me - use Facebook, Google and/or Android. Pulling out of WhatsApp if you use those services is a bit horse>door>bolted. Absolutely switch to Signal if you're a privacy nerd/activist/involved in nefarious stuff. But for most, WhatsApp remains fine.
In other words, don't let perfect be the enemy of good. I'd rather all our dads used WhatsApp than SMS.