I limited to the past 30 seasons. The D was far more often mediocre than elite.
In 2005, they were 1 and done in the playoffs.
In 2006, they had the #2 scoring Offense (#3 D) in the NFL. Lost the SB.
In 2018, they lost a WC game, at home (first one in 8 seasons) to a 9-7 with a mediocre defense and a journeyman backup QB. Elite teams don't do that.
So even though they had a handful of seasons with top Defense, they have been 1 and done 5 times in the playoffs in 8 trips over the past 3 decades.
And of course Bears fans will look back on the legendary Bears D of the 80's. But I can't imagine that's what they are interested at this point. What fan base wants that today? Ask them if they would rather have the #1 scoring Offense or the #1 Defense.
They've witnessed plenty of the latter--as you have pointed out, mostly in futility. They should be hyped about that possibility with a new regime (I'm using the term "new" kindly with Dungy and Frazier)? With a top D and Justin Fields and some OC to be named later? They've seen this movie before: loaded up on D paired with an overdrafted rookie QB.
I don't see it.