A user already has:
- Telegram
- Facebook Messanger
- Instagram that has direct messages
- the good old email, or better, many of them
- Microsoft Teams for company communications
- Discord for communications with group of friends
- the old SMS (that I didn't even know that in some parts of the world were still used, since I receive them only for 2 factor codes, notifications about my card transactions, and spam)
Adding another app is that a big deal? By the way I don't use Signal, but not for the reason of not having another app on the phone, just because I don't know anyone that has it and actively use it.