Two reasons to want Homeworld at the moment:
1. To try and obtain publishing rights for it, so it can be made available online, say on GOG.
2. To develop it as a franchise in whatever direction that is.
Come on, whenever Relic is mentioned or people clamour for a new RTS, people often shout out for Homeworld 3. It's on name alone. Look at Diablo 3. That said, I don't disagree with you. There's really no reason why Relic or Sega would buy it, other than for reason 1, buuuuuuuuuut, I could see Relic, if they really wanted to work on it, trying to twist Sega's arm by saying that they'd be happy Sega not buying it, if they were able to work on an RTS that was a spiritual successor. I don't see it happening.
If Relic were left to keep on making awesome 3rd person shooter/melees, or awesome RTS, I'd be perfectly happy.