If we can do it all on Nostr, let's try. Built from the ground-up, the right way. 
Simplex does secrecy better, just as many other things have done things better than Nostr, at one point or still do. 
If I can use my npubs to log into everything, whilst preserving my privacy, degree of anonymity and security, I'd like that and I know many others will too. 
It's all about giving options.

It might just get other services such as Simplex to somehow integrate Nostr in one way or another (perhaps not identities, but other things). 
At this point in Nostr's development, shooting down possibilities is like suggesting why use Nostr instead of something else. 

For the past decade we've had to install tons of apps for tons of social circles with no interoperability. 
Let's give interoperability a proper go, perhaps without sacrificing the rest, and we'll see where we land.