I seriously doubt this. The Amex plat card offers significantly more value for the annual fee and also has been around much longer than the Reserve card. Lots of people have the Amex card.

Also, remember that the Amex-Delta partnership is reported to be extremely profitable for both companies. I don't see DL turning away the income it gets from all of those Amex swipes, but more importantly, I don't see DL doing anything to turn down the massive income it gets from the Delta branded Amex cards, and this is likely all part of one negotiated deal.

With Sky Club being the only remaining legacy lounge network that the Amex plat card grants access to, I would think that Amex would work very hard to preserve that benefit as they would risk loosing many cardholders were it to go away.