It's a very fair point and while it's just so hard to speculate that far out, surely the estimates for Goff and Donald are reasonably accurate. I would hope they are paying Gurley under a franchise tag in 2020.
Another hope of mine is that they find their cornerstone LT in the 2019 draft. You need some of your key players playing under their rookie deals. Right now it's Goff, etc. By 2020 LT has to be one. You need at least one other starting OL then playing for less than $1M.
I also highly doubt Talib and Peters are Rams in 2020.
Maybe Peters if he really behaves and plays well. But it would not be surprising for those guys to be two-year plans.
In the end, good teams have to draft well, pay their stars and be willing to let some players go and take the comp. picks back. The potential of the 2020 Rams will be in large part based on their 2017, 2018 and 2019 Drafts- plus Goff and Donald of course.