If I'm Houston, I am comfortable throwing away my rebuild for a short window run. If I can get Harden in FA and then line up two big trades for stars, I go for it.
I am not convinced by their youth. Jalen Green is their most talented player but is far from complete. Jabari Smith looks more like a 3rd option than a top 2 dude on a title team and that is if he pans out which is no guarantee. Sengun is skilled and smart but lacks athleticism, can't play defense and doesn't shoot the ball well.
It is a good start to a rebuild but it is not overwhelming talent. It is not Durant and Westbrook and dreams of a decade of title contention. It is closer to the Magic with Oladipo, Vucevic, A Gordon and E Payton. Is Houston's rebuild really so promising that you would be worried about walking away from it? Not for me its not.
A trade package built around J Green should net you a very strong veteran player. A 2nd package built around some combination of Jabari Smith, Sengun and maybe picks could be quite attractive also depending on how much you stack the package with Sengun and/or picks.
Houston could build a very good team around James Harden. Compete for 3-4 years. Restart their rebuild then.
I do wonder if they could get to a title contender via big trades. Maybe. Maybe not. But they should be able to build a very good team. A 50-55 win team. Maybe better.