I suppose but rebuilding a network doesn't happen overnight. Trust me I wish it can be done quicker and faster but it takes time, the right people, permits, licenses, making sure the new network doesn't affect competitors existing networks, series of testing etc.
Sprint isn't just adding LTE panels they are restructuring every piece of equipment including the base stations and fiber cables.
It's something that sprint has to do to keep their network alive. But once they are completely done with network vision and once LTE advance becomes more known and advertised, sprint will already have the equipment in place to launch it. (I honestly don't know if att and vzws current LTE network and equipment is capable of LTE advance to my knowledge its not)
One more thing - sprint is adding LTE on 3 different frequencies
So overall it's a lot but like I said it has to be done.
*just writing all of that made me realize I have to be patient and I know once everything is done it will be worth the wait*