I would imagine with just an nsec, they can't do proper key derivation (like they can with seed phrases) So just having an nsec means their messaging scheme couldn't function. Just a guess though.
Indeed. It is probably worth the trouble of supporting legacy nsec identities to get users in the door And then write a sweet migration tool that automatically notifies your compatriots once you migrate to a proper seed phrase
If I understand correctly, you want Keychat to support importing nsec, which would increase the complexity of Keychat. We will consider this issue once Keychat reaches a stable version. We recommend not using the same ID for Damus and Keychat.
You must consider the cost of forcing new identities on potential users, identities that are an anti feature to most current nostr users