On the other hand only AMEX's lounge access program gives access to Delta. I think UA is taking sides with either visa or Chase (don't know which yet) and DL AMEX. UA probably negotiated this restriction in lieu of just leaving the program entirely like Delta.

I don't know PP's business model, but my assumption to this point is they charge a flat annual fee to you, and pay peanuts for each visit you use. They hope they make out over their entire customer base from lack of excessive usage.

One thing which will be interesting to determine is if AMEX moves to PP, and in doing so they loose Delta lounge access. I imagine they wouldn't let this happen.