I don’t think there’s anything on a personal level that you can give them. I personally have accepted that I won’t convince people to switch so I use WhatsApp. That being said, up until very recently you could use the “hidden in the crowd” excuse, e.g. there’s no way for them to monitor every single thing that goes through them effectively, this has changed with the advance of LLMs.

This might not concern them, but the truth is that nothing you do in WhatsApp is private, the fact that you can read the same messages on two different devices at the same time without having had to input any sort of password is a dead giveaway that the information is stored unencrypted in the servers. This means that every message someone has ever sent on WhatsApp is likely stored on a huge database that can be scrapped by LLMs to find out information on an unprecedented level. Finding every user who has broken the law by for example mentioned using weed or illegally downloading content or finding all of the password for all of the accounts people have sent over Whatsapp is as easy as asking it in English. And the question you gotta ask yourself is “Do you trust Meta not to abuse that power? Do you trust every single employee at Meta with access to this database not to scrape it for personal gains?”.

But realistically that would fall on deaf ears. If you truly are serious about this you should flip the script, e.g. “Signal can do everything that WhatsApp can, AND it’s secure, which is important to me even if not to you. If you decide to use WhatsApp you’re saying that my opinion doesn’t matter, because you have no argument other than inertia. It’s like if a friend of yours developed an allergy to shrimps and you decide to still keep meeting in a Bubba Shrimp restaurant weekly because that’s what you’ve been doing.”