I get your point, but I think "all-time" is a stretch. A number of teams have won SBs with top Ds for that year, but not close to all-time. The Giants D was a significant factor in beating the Pats but weren't top t ALL-TIME. The Rams D wasn't the reason the Rams lost the SB. I agree, top Ds are hard to build, but you hurt you'r argument with "all-time." And top Ds have beaten elite QBs.
Given that, elite QBs are hard to find, and if you want to be consistent, you'll have to argue a top 5 all-time QB is is hard to find. But, that's not your point. Your point, if I read you correctly, is whether it's harder to build a top D or find an elite QB. Personally, I think both are difficult. In fact, I'd say 'rare.' That's why I'm in favor of getting a top 10 QB. Much easier to get one of those.
3 often mentioned are the 85’ Bears, 00’ Ravens and the 01’ Bucs... all three had very average QB’s...all three had one of the top 5 defenses ever.
I’d say that’s extremely elite defenses.