Yea, but then you make everyone rely on 3rd parties bunkers, or push them to run a server 24/7. Also in that way subkeys will pollute relays since they are just made to trigger the bunker and get a signature from the master pubkey 

 Those pubkeys are very low bandwidth. The (personal) bunker listens for one pubkey per client you use.

I want a bunker in my phone. That's always on.