This article talks about the stadium design and says that the roof was supposed to withstand winds up to 120 mph. The desire to make Reliant more of a multipurpose facility than a pure football field (it hosts more rodeo events per year than NFL games) seems to have a lot to do with why it wasn't set up to have an open roof indefinitely. The floor without the drainage system is probably a lot more useful for non-football events.
Rice Stadium may be the next best option but I don't think it's a real attractive one for the team. Even if every Reliant ticketholder gets a seat at Rice, Rice would clearly be a step down in quality for the fan -- sightlines, benches instead of seats, no luxury boxes (right?), fewer concessions and bathrooms, less parking and access for traffic, no video screens (right?) etc. The team would need to do something to make up the difference to the fans, not to mention the hit it would take in lost signage, video, and other in-stadium advertising revenue.