Every time I've read about airlines talking about hidden city bookings, aka when they've had to defend in court why they cancelled an account, or cancelled a ticket, etc etc - basically the serious times.

Every time, they reference that the person is getting a cheaper fare/ticket by cheating the system, and that seems to be the basis of their argument.

Since you're not getting a cheaper fare at all, FP credits etc, they may very well not care for this reason.

Also - just a thought. Couldn't you do a free SDC at the airport day of because it's a latitude FP to just drop the last segment? or same problem there? Because that would still preserve the upgrade.