Trying to trade for them isn't completely crazy, but offering RB2 value certainly is. They don't have the best schedule and they are on Bye next week. Look for a defense on the waiver wire that has a good matchup next week and offer to trade that defense to the Bears owner, who is going to need a defense next week. I dropped the Bears this week to pick up the Chargers for that reason. The Bears are a great defense going against a great offense - look at Thursday night if you want two examples of how that might turn out. The Chargers are an average defense going against a bad backup QB. Bears or Chargers this week is pretty much a coin flip for me, and the Chargers get Oakland at home next week, while the Bears get nobody next week.
If you want to win at fantasy football, stream your defense. If you just want to root for your team, I got no problem with that. But having your favorite players on your team can be a double edge sword - you will hate them even more any time they have a bad week. If you love them when they win and get all depressed when they lose, don't pick them up. If you just love the Bears all the time no matter what, then by all means pursue a strategy to get them at a reasonable cost. But even the flat out guaranteed best DST in the league is not worth an RB2.
You might even find the Bears on the waiver wire at some point this season. Read this article every week for suggestions if you do adopt streaming, and especially look at the bottom of the page for suggestions for Week 5 if you want to try to trade for them then, since those are the defenses you will be offering.
https://subscribers.footballguys.com/apps/article.php?article=2018-rent-a-defense-week-4
Here's my question:
https://forums.footballguys.com/forum/topic/770875-possible-mahomes-trade-whir/