AMEX is making their own "Centurion Lounges" at various airports just for AMEX cardholders, so that may be the plan for the future. I stopped at the one in DFW, and it was much nicer than the US Airways club at PHX I have been to in the past. If they make enough of the new lounges it will work out pretty much the same.
The problem is they will never make enough. If I'm flying through a major airport chances are good that I am flying on the airline that uses that airport as a hub (or at least is a major user of the airport) and therefore has lounges that I would have had access to. There is just no way that AMEX will be building multiple lounges at all major airports to service almost all of the gates. Partnering with airlines gave access to hundreds of lounges. The priority pass select's "600 lounges" are primarily outside the US. While that is convenient if you frequently travel internationally, it doesn't help you much if the majority of your traveling is domestically. I don't know what is driving this change, but it is going to be a major change. I agree with Zab, if I were paying the $450 annual fee I would consider canceling. Since I'm not paying for it, it's worth keeping for now.