It is a nearly $0 expense item. It incentivizes continuing spend from someone who might have become a "ho" (I did mean "free agent, no?) - all other things being equal, such as "I could fly on UA or AA or their partners as a plebeian at the same cost of getting some STE benefits." As a UA MM that rarely flies UA or *A anymore, having *G in the back pocket is a nice thing to have, and occasionally drives revenue to UA, at essentially no cost to UA. Maybe I occasionally get a Y+ seat, or a visit to a lounge. When I don't fly UA or an *A partner, it costs UA nothing.

So, if DL has a bunch of MMers that don't fly DL or ST, it costs DL essentially nothing. If they fly DL or ST occasionally, it may cost just a little bit, far offset by the revenue received. Given the current state of affairs, it may cost DL an occasional Y+ seat, an even rarer F upgrade (both no cost items - those seats will be given away for free to someone), and occasionally, an extra free bag.