While many are still using WhatsApp, it has taken significant reputation impact and users are dropping. Potential is only for the limited user group. It can be only changed by changing owner in general.
From my experience (Europe), not that many users actually left it. Sure, many people constantly complain about privacy issues but only a few very tech-savy left for Signal. And even they cannot leave Whatsapp as it's the de-facto standard. I don't know a single person who has a mobile phone and isn't on Whatsapp at least once a month.
Same. I prefer Telegram, and a few of my closest friends also use it, so I use it the most, but even my contact list on WhatsApp is at least 5x longer.
The most of my family and friends have swapped to Telegram. Almost all local uni students as well.