I do agree the Bears could use better back up QBs, but I feel like that could be said about almost every team in the NFL at this point. I also think that given the state of the team when Emery left and limited draft picks in multiple years, there haven't been a lot of opportunities to draft a QB.
2015, terrible draft for the Bears, but it's hard to argue that they should have gotten a QB. You got guys like Garrett Grayson, Sean Mannion, Brett Hundley, Trevor Siemian and Bryce Petty drafted in this draft. I would argue they're all pretty garbage and our coaching staff would not have developed anyone of note at that point (I wouldn't trust Fox to have done shit with a developmental QB). Also, the team was a dumpster fire and needed talent everywhere, so BPA should have been the draft strategy going in.
2016, definitely more QBs picked, I could see this year being a better year to pick a QB. You have Goff/Wentz go 1/2 in the draft and you have a bunch of busts like Hackenburg and Lynch, but the Bears probably could have replaced one of their 4th round picks for someone like Connor Cook or even better, pick Dak Prescott (though Dak would be a starter, not a back up and there's no telling how he would have turned out with Fox). The rest of the guys picked after aren't really much of anything and I wouldn't pick any over Daniel. You have like Sudfeld, Kevin Hogan, Jeff Driskel (just to name a few that were mentioned during the draft lead up), all of them suck. Also, the number of needs for the Bears team as a whole probably limited the Bears with a luxury pick like a developmental QB.
2017, we drafted Trubisky. After the top 3 picks, you have Kizer, Davis Webb, C.J. Beathard, and the GOAT Nathan Peterman. Beathard showed promise but Kizer and Webb suck. Peterman is the greatest player ever, but I don't think the Bears could ever handle him so he'd be a wasted pick. Team's finally starting to come together, so at this point, I can see picking a developmental QB after this season, as the rest of the team starts filling in and we actually have talent and depth across multiple positions.
Last season, none of them have done anything of note after the 1st round. I remember people liking Luke Falk but that's about it.
This season, Bears with very limited number of picks, so probably not the best time to pick a back up QB.
Overall, just given the timeline and the picks available, unless you go the FA route (which we did), there's not a lot worth picking for drafted QBs, other than Dak Prescott, who would be a starter, not back up.