Well, based on the e-mail, at least it seems that NW has set a hard & fast rule, which even if you're 1 mile short, they won't bend. And there's the part of me that thinks that a threshold is a threshold... you don't cross it, you don't get the rewards.
However, customer loyalty should have its privileges. If the OP has been Plat for several years, and misses this year by 2,000 miles (perhaps with some extenuating circumstances), and has never asked for a comp or elevated status, I would think that NW should consider the request.
One reason I stick with providers like Verizon (cell phone) and Citibank (primary checking), is that when I need something from them (i.e. fee waived, compensation for errors or other situations), they tend to look favorably that with all of the competition out there, I've stuck with them.