ah there it is....there's the irrational takeaway when making a comparison from year to year performance.
teams can go one of two directions each year: better or worse. there's gonna be top 10 defenses that are gonna be worse and up&coming defenses that are gonna be better. some of us believe the bears are of the latter, to the point where we can be top 10.
Nothing irrational about it. However, stating a team that was "almost" top ten in total yards allowed does not back the idea that they will somehow be a top ten D this year. As you just stated, no team stays static from year to year and I am stating the same thing. Therefore we are on the same page in that regard.
Where our opinions differ is I do not feel that you can be a top 10 D without a good secondary and the Bears have a lot of holes, if's, and maybe's at corner and safety. No matter how good your front 7 are, they cannot realistically get to or hurry the QB every play. Hard to keep the yards against down if the opposing QB is firing the ball all over the field to wide open receivers.
Also, if you are simply looking at how the Bears were "almost" a top 10 team in yards allowed last season, I then wonder where the opposing teams started from? Did they start every drive from their own 20 and the Bears stifling D create a bunch of 3 and outs? Or, perhaps, the Bears had a QB that liked to fumble and throw the ball to the opposing team giving them excellent field position where they did not have to have long scoring drives? I would have to go back and look at the stats on that but I would hazard a guess that it is the latter. Oh and according to the NFL's website, the Bears were ranked as the 18th "overall" D in the league last year.
To top that off, if you are looking at last year as an indicator of how good they will be this year (which is fine for comparison purposes) you also need to look at the strength of their opponents O this year vs last. If they are playing better teams, even if the Bears are better on D, their numbers may still go down.
That said, I hope they do end up as a top 10 D. I just don't see it although I would love to be wrong.