Last year the Bears weren't going in the 5-7 range...in my main league the Bears were drafted in the 15th round and this was on Labor Day after the Mack trade. Even if you account for homer inflation that puts the Bears in the 10th round range at best, making a 3rd round pick on the Bears a complete waste.
As for this year they are going in the 7th-8th round range and maybe earlier if there is a Bears homer in the league. But lets assume it is the 7th round. I wholeheartedly believe that the Bears Defense will be good this year and likely finish in the top 5, but last year the Bears put up incredible numbers that likely will not be repeated. On top of that historically Team defenses do not repeat as a top fantasy defense. So in the 7th round you are drafting someone to start in the flex or your first bench option. Per ESPN ADP in the 6th to 7th round players like Calvin Ridley, Phillip Lindsay, Allen Robinson, Josh Gordon, Robby Anderson, Tevin Coleman, Austin Ekler, and Duke Johnson are all falling in that range.
I would much rather draft any of those players instead of a defense and wait and stream defenses for the season than draft a defense that early considering defenses can be a very volatile position.
Also when you take into account injuries, a defense in fantasy has a better chance of weathering an injury to that unit, but if your starting WR goes down with injury you have to replace them. I would be much more comfortable knowing that I had any of those WR's listed to plug in, 2 of them being the #1 WR's on their teams. The drop off between drafting a defense in the 7th and drafting one (at the earliest) in the 12th is marginal.
Now take into account historically the #1 defense from the prior year regresses in terms of fantasy the following year. Her are the #1 defenses and their drop off the following season
2017 - Jacksonville put up 203 pts, in 2018 they dropped to #12 and put up 117pts
2016 - Chiefs put up 166 pts, in 2017 they dropped to #12 and put up 123 pts
2015 - Cardinals put up 186 pts, in 2016 they dropped to #4 and put up 144 pts
2014 - Eagles put up 177 pts. in 2015 the dropped to #12 and put up 134 pts
2013 - Chiefs put up 207 pts, in 2014 they dropped to #18 and put up 118 pts
2012 - Bears put up 222 pts, in 2013 they dropped to #18 and put up 117 pts
2011 - Niners put up 184 pts, in 2012 they dropped to #7 and put up 151 pts
2010 - Patriots put up 187 pts, 2011 they dropped to #9 and put up 142 pts
Taking a defense early because they finished #1 the prior season will end in disappointment for that fantasy player. The team may have a solid season but I would rather pass on that volatility than hope that the defense produces similar stats like the year prior.
tl;dr - Players are a lot more predictable than a defense, don't try to bank on last years success translating to similar success in the current year.
