I'm afraid I'm on the other side on this one. With all of the ways to earn miles, it is a trivial issue to get something deposited into your account at least once a year. If the airlines feel they need to cut back somewhere, for a frequent flyer (and that is who the programs are for), this should have little or no impact.
Every store I know that has a frequent buyer program has limitations on how long your credits last. In this case the airline is not putting an expiration date on your miles; they just saying you have some activity in the account once in 18 months. I guess I am in the minority but I just don't see that this is a big issue.
On the other side of the ledger, is Perkins making a big deal about the enormous increase in the ability to earn miles on US with the merger with HP? For US ff's, I would think that enhancement would be worth far more than what this new limitation takes away.
Once has to pick which battles to fight - for me, this would not be one with a high priority.
I do agree with tusflyer, however, that notification of changes need to be made with plenty of advance notice.