Obviously there's no efficiency in consolidation if they still have to operate all existing satellite's, ground stations, etc. However, I just don't see how they could pick one or the other. It's not like moving to a new cell phone technology, where you can just go to the store, get a new phone, swap your phone number, and be done. If everyone started using say Sirius hardware tomorrow, you could say in 7-10 years, maybe there won't be a lot of XM in use, and it can be turned off. In the short term, I don't see that happening.
That's why I think the only realistic solution is to consolidate down to a common programming (common channels), which would then be broadcast over both systems.
Jeff