Not a fan of Meta, or whatsapp, but I have to use it as it's a more universla messaging app, and RCS support in the UK for iPhones is not existant. No one I know uses imessage in the UK other than posters on here claiming they do. Making it default will maybe stop siri sending a text when I say "send a whatsapp...". Unlikley though as Siri sucks.
Siri did the only intelligent thing I've seen it do the other day and when I said "Send a message to (Contact)" it asked me "You usually use Whatsapp to contact this person, shall I use Whatsapp for (Contact) from now on?" I answered "Yes" and now it just defaults to Whatsapp for that guy.