Correct me if I'm wrong - from the reading I have done WAAS is a requirement in the States for LPV approaches, isn't it? Never mind that the larger airports still have ILS the FAA recognized the potential to establish instrument procedures at a lot of underdeveloped airports without costly ground based aids, something that is completely unheard of in Europe
Regardless, GBAS is currently only part of the GLS implementation (where at least in the 737 you would even manually select the GBAS station on the MMR) at this stage, enroute ANPs of 0.05 even without WAAS are usually not a problem anyway.