You do have to assume that they would want to leave 4.5 as the final upgrade for some of these older devices even if they were able to get 5.0 running quick enough on some of the older hardware and realistically with memory constraints that task wouldn't be an easy one either. I expect that if you aren't already running 4.6 or newer on a gsm blackberry then you won't be seeing 5.0.

CDMA might be different since it has less models anyway and they haven't had their version of a curve 8900 in the meantime either.