The value proposition depends a lot on what you are looking for in a lounge. I haven't been to a DL SC recently but from what I've seen they have nice food, good selection of free premium beverages, and many are nicely updated. Many are also crowded, membership is quite expensive, and DL is implementing access restrictions to deal with that crowding. They have basically removed access for their own medallion members traveling international long-haul Y, among other restrictions. Personally I wouldn't be happy to pay much more for lounge access if I can't actually use it as advertised as much as I'd like.

The ACs are just quiet spaces where I can sit for as long as needed, work, read, use wifi. The food isn't terrific but it's decent. You can assemble a light meal between flights. Sometimes I'll grab a nice sandwich or salad in the terminal, bring it to the AC, and supplement my meal with the snacks. I greatly prefer this over eating at a sit down restaurant in the terminal, where the food will be mediocre, overpriced, tables crammed together, and the experience rushed. As others have mentioned, availability of showers is a nice perk, and many locations still have outstanding AAngels to help out with reservation issues. Overall I'm happy paying what I'm paying now for the experience I'm getting. If AA were to announce further upgrades on food, raised prices more, and then had to contend with overcrowding with diluting how much I could use the AC, I'd reevaluate whether membership was worthwhile.

Lounge quality varies quite a bit. CLT is some of the worst, and I haven't been crazy about PHX. But some like DCA-E are quite nice, and other hubs like ORD, JFK, PHL, and my home BOS are quite pleasant.

And mind you, I'm old enough to remember when free food in the AC was the pretzel mix and a basket of apples. This is one area I think AA has gone in the right direction.