When Tesla was desperate to get cars sold and delivered prior to the end of 2015 to get more sales on the books, they were calling everyone trying to sell them a car, even "negotiating" on anything other than the actual price of the car (trade in values, items included, etc), they would have been really smart to announce "85 kwh and rwd options will no longer be available in 2016". For anyone that needed a battery with more than 70 kwh and was holding off, likely would have sped up their purchase to save $8,000.
I think you're neglecting one major point: Tesla wanted to stop manufacturing 85 kWh packs, and probably did some time ago. They probably continued taking orders for cars with 85s until they sold out of the 85 packs, at which point they cut off the sales of the 85s. Tesla couldn't really say "Order before February 8, 2016 if you want an 85 pack", in an effort to increase sales, because Tesla didn't know how long the 85 packs they had in stock would last, and, to be honest, they probably didn't really care.
The only thing Tesla could have done along the lines of what you were looking for would have been to announce the discontinuation of the 85, while at the same time announcing that there were a limited number still available, but that when they were gone that would be it, so if you want one, it's first come, first served. The problem with that is that it would appear really cheesy, and not in-line with what Tesla is trying to portray.
As NSXROX pointed out up thread, the writing was definitely on the wall. I think Tesla handled this just fine.