Since Signal also uses phone numbers as account ID, you'll give that to WhatsApp at least. Then they continue to track metadata of the messages sent between you and your WhatsApp contacts and will be profiling you.
I don't see how communicating with any Meta service isn't compromising privacy. I'm a Signal user and won't connect to WhatsApp.
At least WhatsApp won't be able to monitor app usage patterns or scan my filesystem and contacts. Seems at least a bit better than the alternative assuming you have contacts who insist on or have a strong preference for WhatsApp
But for that signal need to comply with whatsapps request to integrate signal messaging with whatsapp api according to their terms and conditions. (Probably share details from signal users, which affects privacy) otherwise it would be unable to sent cross-platform messages.
Also it breaks encryption at all cost.
According to the article, WhatsApp requires the Signal Protocol for message encryption.
Signal is the single third party that shouldn't have a problem with that since it's been using that protocol before WhatsApp adopted it, too (they hired Moxi himself to help them do it, remember?)
Signal and whatsapp is developed from same developer. They only thing they have same is encryption protocol and nothing more. If users of whatsapp need to contact with signal users they need to agree with certain toc and rebuilt the app. Practically, no signal user will wish to communicate with whatsapp users bcz of lacking encryption and data privacy issues. I dont believe this third party chatting idea of whatsapp won't work bcz signal and other popular messengers like matrix,tg is for privacy and security. Also i don't think that no platforms will support this idea of whatsapp as chrome proposed FLOC.