I would say that DM is far from useless. The C+ at booking is a huge benefit for the extra legroom, as well as the choice benefits.
Some things have been significantly eroded over the years, such as removal of SC access/membership unless you burn multiple picks for a exec membership... and also the fact that DL has gotten quite good at selling F seats right up to day of departure.

There are certainly a lot less upgrades being handed out, as well as more seats being held back for DL to try to sell them inside the T-5d period instead of processing upgrades.

If DL has reasonable upgrade offers then I bite on them also... but plenty of times I didn't take the overpriced offer and ended up 1/xx on the upgrade list for 0 seats... so they are finding suckers to give them a pile of cash to sit up front.