Well...we don't have much precedence on this one.
9780 users got shafted quite nicely. 9700 users at least got the upgrade to 6.0.
Point is, I hope RIM doesn't shaft us 7.x users like they did to others.
If Apple can support the 3GS (which, based on my use varies from mediocre to painfully slow with iOS 5) for this long then RIM shouldn't be too hard pressed to keep at least things like the 9900/9810 going for one more run.