It depends on what you mean by "en masse." If you’re referring to the average user who doesn’t want to run their own Bitcoin full node, etc., then...

Spaces uses DNS, so you could easily leverage solutions like Fabric behind a mainstream resolver (e.g., 1.1.1.1) to benefit from its massive caching layer. Implementing this isn’t difficult, for example, by using SIP-02 (see: https://github.com/spacesprotocol/sips/blob/main/sip-0002.mediawiki).

Users don’t need to trust 1.1.1.1; they can verify the entire protocol state with a small zk-proof.

Spaces guarantees name ownership in a permissionless way, functioning somewhat like a certificate authority—but that’s where its role ends. Solutions like Fabric manage the records off-chain.

since the inception of the internet no email addresses have been defined directly on the tld so practically no collisions sip-02 also attempts to make this as painless as possible to integrate even into mainstream resolvers

do we even need sub sub spaces? what use cases do you have in mind?

yes checkout the current proposed draft https://github.com/spacesprotocol/sips/blob/main/sip-0002.mediawiki