Yes. I don't see Signal and Matrix in direct competition, just like WhatsApp and Slack are not in direct competition. The technology is very similar (main difference seems to be the size of chat rooms), but the use case and marketing is very different. Signal/WhatsApp is for casual mobile texting, while Matrix/Slack is for working.
I think Riot is a better comparison to Slack than Matrix is. Riot is essentially the Slack experience built on the Matrix protocol, but Matrix can certainly work just as well for clients that present a Signal/WhatsApp/iMessage/SMS-style interface.
It might be the case, but I should would prefer being able to stick to just one messenger in the end. And I don't see why it wouldn't be Matrix as opposed to WhatsApp. (If we ignore the networking factor, of course.)
