1. It must be mandatory and coded by all relays and all clients. Otherwise, users will see broken experiences everywhere (things appear here as official accounts of a brand but not there, etc).

2. Encryption and decryption is impossible with subkeys. For instance, we can encrypt DMs for all subkeys, but once you do it, you cannot revoke that anymore.

3. Replaceable events and all the indexing around it now must consider delegated keys whose authority can change over time on a simple re-broadcast. The entire indexing now needs to use the DB as a source for the index itself. It gets extremely complicated.

4. Now compound that complexity with the fact that we don't have a time chain on Nostr and things can appear in the past, future and different relays can and do have different versions of what's authorized at the same time. 

It's mess, on top of mess, on top of mess. 

All because we use raw pubkeys as the main address and not a time-resolvable DID for instance.