We've talked to owners who really liked their GPS units, but they flew only one aircraft and had spent hours and hours learning that receiver's nuances. We haven't heard from any renter pilots or CFIs who felt comfortable with having to learn multiple operating systems for a flight-critical component. No longer will GPS be the province of the early adopters, the technically proficient, or the gadget freaks. Everyone will have to learn, and increased complexity is not the way to bring new people into the aviation business. Simpler is safer.
Some training is essential.
After all, you didn't get to be competent without some effort on VOR and ILS, and it is unrealistic to adopt a completely new way of doing business without some level of training. Likewise, it is unrealistic and dangerous to expect pilots to learn diverse operating logic and maintain proficiency on each unit. It's like having to learn several word processing programs. Historically, when new technology is introduced, there is a learning curve for both designers and pilots. A primary nav system is an excellent place to remember the human factors lessons of the past and to not rely exclusively on training to save the day.