The gap between $15K MQDs for Platinum and $28K MQDs for Diamond is just huge, almost double. I can easily make Platinum with my ordinary Reserve card spend and ordinary flying activity. But there is no way I'll meet $28K without a significant change in work or life circumstances, which I have no reason to expect. Accordingly, once I reach Platinum, as I usually do every year in about September, I sort of become a "free agent" (though I'll generally choose Delta so I can enjoy the perqs of status),and also might tend to defer optional and flexible travel until the next calendar year.

My suggestion to Delta might be to hand out some sort of perq to those reaching, say, $20K or $22K. Not a separate status tier, but maybe handout a couple of additional RUCs or a couple of SkyClub guest passes, or something like that.