Maximum nr. of users on Ethereum

This is out of my scope, but there’s a consensus mechanism and there’s a transport mechanism.
In a simpler scenario Bitcoin could (maybe?) use the Safe network to communicate (like it can use Onion), but it would be very difficult (if not impossible) to use the SafeNet consensus mechanism for Bitcoin (very different things).

The costs aren’t coming from the comms, but from transaction validation (in case of Proof of Work). You can’t combine PoS with PoW without making major changes (to put it mildly) to either Bitcoin or MaidSafe. I think Ethereum plans to change from PoW to mixed, but they’re designing this flexibility into their network now.