The way I remember it is that Seattle didn't want to shell out hundreds of millions for a new venue and the OKC owners only bought them because the previous owners didn't want to pay either. With the state Old Trafford is in who's to say the same couldn't happen in 10 or 15 years?
Seattle didn't agree with Schulz on a new venue which is partially why he sold but moving to OKC had nothing to do with Seattle not being able to sustain a team and everything to do with the owners wanting to move their franchise to their hometown. Think about it this way OKC didn't offer a new arena and the current arena isn't a 500m sexy arena, he used that as an excuse to renege on the good will agreement to stay in Seattle which was predicted at the time.