I don't think it's completely insane. I'm no fan of Mayhew/Lewand, but I'll admit they were dealt a shitty hand.
The worst time to be a terrible team happened to be when the Lions were at their worst (shocker, I know). Escalating rookie contracts were getting out of hand and the Lions had top 2 picks in 3 out of the last 4 non-rookie cap years. The future cap was most certainly going to be screwed by this (and it has been).
Now Mayhew/Lewand in 2015 are responsible for all the bad/overpaid contracts and multiple restructures, so 99.9% of the blame goes on them. They needed to work some cap magic to compensate for the insane rookie contracts and they haven't really succeeded.
Millen was responsible for creating an unstoppable losing behemoth that was the late 00s Lions. Bad teams led to high picks, which led to insane rookie contracts, which led to an out-of-control cap clusterfuck...and Mayhew/Lewand didn't do enough to control it.