From what I know Signal and Threema refused interoperability with whatsapp because doing so would allow Meta to get their hands on a lot of data.
Personnally I wouldn’t mind interoperability on an opt-in basis so that the user would have a choice.
For now, my solution is to mainly use Threema and Signal.
I have Whatsapp business for when I have no choice. With Whatsapp business you can prevent people from adding you to new groups and you can have a message telling individuals to reach you by Threema, Signal imessage or email instead of Whatsapp.
This has allowed to still be on the groups for which I have no choice, while almost leaving Whatsapp.