How is it horrible? Detroit would be getting a side-grade on D...lose a salary that hasn't been productive for them and won't be around when they're competitive again, and have money to re-sign Mantha/Larkin.
Let's break this down. Detroit, a team who, regardless of not wanting to admit it to their fan base and lose out on ticket sales, is rebuilding, gives up:
Green - Will fetch a 1st+ at the deadline. Has to be moved because the team doesn't have the money to re-sign him.
Nyquist or Tatar - Can likely fetch a 2nd+, maybe even a 1st if you find a team desperate enough at the deadline.
2019 1st - Guaranteed to be top 10 with or without Letang. And I don't care if there's a protect or defer option, 2020 will be top 10 as well.
To acquire Kris Letang, a 30 year old injury-prone defender who
just had a stroke. Best case scenario, even if he has a long and healthy career he's of no use to Detroit as he'll be riding off into the sunset by the time they're competitive. Worst case scenario, he continues his trend of injuries and health problems and is of little help to Detroit while they've sold off draft picks and assets that could've been used to acquire draft picks.
I'm not saying Ken Holland wouldn't do it, because I honestly can't figure out what the hell that guy has been doing for the last 5 years. But if he did, that should be the straw that breaks the camel's back to send his ass packing.