And do you recall Heroes of Might and Magic V? In theory - made by a different team. In practise - great heir of HoM&M 3, with all its best advantages and without its disadvantages. It was different, although it was made in different times but it is still an amazing continuation of this saga.
There's no need of bringing entire team back together. Like in Deus Ex: Human Revolution - few man from the original team can be main designers and the programmists, artist etc. can be people of different studio. ^-^
Also reason to design HW3 can be really simple - everyone has their dreams and there can come time when Relic will try to ressurrect HW.
"Hoping for Homeworld 3 is like Hoping for Outpost 3." Maybe you're right but if waiting for Homeworld 3 would be like waiting for Duke Nukem Forever... You see... 10 years after, it realises.
Homeworld 3 is a ghost. If you believe in it, it will come true.