All valid points. Still, most of Delta’s revenue comes from AnEx transactions, and every corporation must demonstrate growth of revenue every year within the rules of the game.
True, there are many customers who are used to put charges on AmEx to accrue Skymiles for family vacations. However, even they will make an effort to spend more (even ask their non-playing friends and relatives to put transactions on their cards) to become gold or platinum. With rollover they could be incentivized to do that every other year when they have a chance of making it.

A gold medallion is extremely cheap for Delta, unless they fly abroad and use Skyteam lounges with guests. The revenue effect from many of those customers spending beyond their means on AmEx is huge.

Most of the Flyertalk members are diamond and platinum, and they tend to forget that there are many more customers with lower tiers, whose total spending adds up fast. Meanwhile, they receive very minimal benefits, while being forced to pay for Main instead of BE. I would bet that the profit margin on them beats the profit margin on diamonds whose employer pays for their Delta tickets anyway.