I've been trying to find information on this. For some reason I recall there being a long-time contract between the FedEx Forum and either the NBA or the Grizzlies for a large number of years. I want to say that if they were to be bought out, their commitment to play at the Forum has to be bought out as well - which isn't cheap. Basically, anyone interested in buying the Grizzlies and moving it would have to pay a premium to get them out of the the Forum deal. I'm not saying that would never happen, but I assume that prospective buyers would look at multiple other options beyond the Griz since they would have to pay a good deal over the market value of the franchise itself to move it.
You're right though, nobody gives a **** about Grizzlies basketball. Just like Memphis Tigers football, the locals will support it only in good times. But, you can forget about it in bad times.