The reason why there couldn't be a 10 year crossover might be economics. The transition is a federal mandate. It came about as a money raising idea by our government.
The part of the bill we hear about is about the portion of the spectrum that is going to be given to first responders and emergency services so they can communicate more effectively.
The part we don't hear about is that the big money the government is looking for comes from auctioning off the balance to cell phone companies.
The government doesn't want to wait ten more years to collect!