A lot of people don't like the stress of changing jobs and possibly moving house to boot.  These are often depicted as lifers and as such they can be abandoned by the management because they are available to prop up the older systems that newer employers won't touch.  As a result their skills rust and their career stagnates.

This isn't loyalty this is the usual reward for not job hopping. 

Failing to retrain and remotivate long serving staff is a usual management failure - many long servers are undermotivated and therefore less productive than staff that have been with the company for, say, a year or more.  But they are still more useful than college starters or job hoppers because they have more business knowledge and committment. 

If companies committed the same resources to training the long servers as these other groups they would reap proper long-term benefits.  Whereas training job hoppers or graduates is pointless as they will probably move on within a year anyway.  Retraining long-servers will actually pay for itself.