Here is a complete list of teams I would bet serious money will have the same starters in 2020 as they will in 2017:
Your entire scenario has huge problems. First, the 2020 season is the fourth season from now.. Teams aren't looking for their 2020 QB, they're looking for their 2017 QB. (other than maybe New England). Teams just don't look that far out, partly because of "win now" but also (and correctly) because too many things can change in that time. At most a team might draft a guy to sit a year, but almost never more than that.
Also, you are pulling a mini-PFF there. Putting out a subjective hypothetical, then using a percentage derived from that to make it look like math. Unfortunately it isn't. Not only is is just your opinion and nothing more, but it's also an oversimplification of the situation.
First, you completely overlooked Tennessee, GB, Lions, Ravens and NYG (Manning has at least 3 more in him). I could easily argue Philly will likely stay with Wentz. Miami isn't going anywhere. Cousins is either going to get resigned or go to SF, WASH won't tag him next year, so while he isn't lined up for 2018 yet, he's not really that much of a wild card (you could almost count him twice as he's probably keeping 2 franchise from moving in FA). Between Bridgewater and Bradford, Minnesota isn't looking anywhere else any time soon. I would also doubt Alex Smith goes anywhere in the next few years. Just with that there are 9 (10 if you include Cousins even once) more teams not looking any time soon. So, about 18 teams not looking anywhere this year or next. Suddenly your 75% looks more like sub 50%. Then add on the teams that recently drafted guys that will probably be given some time in LARams and Denver (and possibly NYJ, scuttlebutt says they actually like Hackman). You can also add on people who aren't looking to sign a big FA because they aren't sure about their QB like Jax. Oh, and also add on people who aren't going to sign a FA to be the future because they have their QB if only for a couple or few years - Chargers, NO, NE, PIT. Add in the Bears who already signed Glennon as their bridge. Finally, none of the FA QBs is realistically seen as a potential answer, at best they are a bridge, so people don't sign them as an answer unless their situation is far worse so even a situation like Buffalo they might sign Tyrod back as the better option.
So, about 26 or more of 32 teams aren't really in the market for a "Big" FA QB. Or, subjective-mathly over 80% NOT looking for a QB. Heck even AZ is probably waiting until next year.
You're looking at realistic openings where you have a chance to sign this year and become a real starter at what, the Browns? Texans? Possibly Buffalo. And I say Browns only because this QB class looks lame. They might feel differently.
Teitan is right, this is a bear market for FA QBs, and we haven't seen it like this in quite some time.