While I agree with the overall point youre making - that making Stafford one of the highest paid players in the league will hurt Quinns ability to improve the D, I certainly don't think its impossible.
You mention Seattles D dropping off - and it reminded me of an article I read a few days ago on ESPN about how Seattle has been able to maintain an elite D for the last 5 years. While I wouldn't say its as 'dominate' as it was 3 years ago when they won the SB, its still damn good, even after they paid Wilson.
http://www.espn.com/nfl/story/_/id/...nse-dominated-nfl-five-years-running-nfl-2016
Also, its not like Stafford is on a (post lockout) rookie contract, where he is vastly outperforming his value. He has a cap hit of $22mil this season, which is only $2.5mil less than Lucks new contract pays him per year. Even if the Lions make Stafford the highest paid player in the league, the cap hit will be at most, $3 mil/yr more than his current deal. Thats a lot, but certainly not debilitating.
The last thing I can say is that everyone mentions how they wish we had a dominating D similar to the 2014 team. But its easy to forget that we had 40% of the salary cap tied up in 3 people...Suhs cap hit was $22mil, Calvins was $13mil, and Staffords $16mil. Sure Suh was a big part of that D being dominate, but the rest of the D was pretty damn good too, and they were able to assemble that group with pretty much pocket change.
So I think its possible to field a competitive D while still paying one guy insane money.