Nagy has yet to be truly defeated by more than the last play of a game, a 2 TD 4th quarter, or a bounce here or there. If the Bears stay this hard to beat I'd say the true figure is between 10 and 25% somewhere. And yes...I think that is a very good and very high number given the competition of the NFL, and it would be way higher if Trubisky could throw left accurately.
I see where you are going, but there are so many moving parts. Here's just a few items off the top of my head that can impact how 2019 goes:
Year 2 of a new offensive system - in year 1 of a new system typically things goe well. Teams have limited info to operate off of and with only a week to prepare (as your team has only shown a portion of its stuff) the offence is at a significant advantage. An offseason of defensive coordinators picking apart tape typically helps them be more prepared. I saw it first hand with Chip Kelly and I also saw that with McVay. Still good, but some regression happened. We'll see how well Nagy continues to evolve. Is he Mike Martz or Andy Reid? I'd lean towards the ladder, but you never know.
Health - a couple injuries in the wrong spot can tank a season. It's unrealistic to assume everyone can play every snap, but the real impact will be who gets hurt.
Opponents - strength of schedule is a horrible thing that ESPN created to fill a dead time of the sporting year, but there is merit in how well your opponent is playing when they take the field. You can play the worst team in the league, but if that's their best game of the year you'll have your hands full. Same can be said for good teams. If you play them in a slump they are very beatable.
Peak too early - Every year after week 4 or 6 there's a power ranking and it's very rare for whomever is on top to be hoisting the Lombardi in Feb. It's just not realistic to assume teams can maintain a high level of play all year long. Anyway, early season success can end in an uninspiring one-and-done come playoff time.
Division - Rogers got hurt and the Vikings regressed. Add in that Detroit might be better under year two of that rocket scientist fraudster who has the audacity to put a pencil behind his ear while working with laminated play sheets!!! Anyway, I'd say all 3 of your division rivals took a step back. It's wishful to assume this is just a start of a 10 year trend where you keep getting further ahead each year, but we probably know that's not the case. I know you hate the rivals, but you have to say they are better than Dolphins/Jets/Bills of the past 15 seasons. I think it was the NFC South that had something like 5 seasons where the worst division team was the champ the following year. In competitive divisions we should expect turnover at the top spot as the target is bigger.