Improbable because these credits come from the merchants, not from American Express. Merchant-specific benefits are always temporary, and to my knowledge American Express has never reduced the annual fee on any card, BUT they may offer larger retention bonuses in the aftermath if they can see that you actually used the benefit.
I have to believe that this Dell offer is different from the "get $x off $y" Amex Offers. I'd be surprised if Amex isn't meaningfully participating in this one. It's just a strict credit with no minimum purchase. I can't see why Dell would possibly fund all or even most of that.