This was also my first reaction to this "problem", did quite a bit of research on this to prove someone wrong, I ended up proving myself wrong.
For running plays to be more valuable than passing plays, you'd really need to have this never before seen dominant rushing attack for it to work... Power running games dont benefit from smaller defenses as you might think. I mean Ricky Williams was much better in the NFL than in the CFL, and defenders over here are much much smaller than the NFL...
Another thing is, if Defenders are lighter, your offensive players will have to adjust and get smaller too. Do we really think bigger slower players on offense have that big of an advantage over a buch of speedy guys on defense? Maybe, but I dont think the relationship would be linear and at somepoint would just plateau, where the strategical gain still wouldnt be enough to make it more valuable than passing.
There's actually a way that the running game might make a come back: Rules change.
The rule change back in 2004 really crippled the value of running the ball, this is the one thing thats keeping it from a viable option. Now this is only speculation but I wouldnt be surprised if the NFL actually tried to bring the run game back by changing some rules. This is heavy speculation but I ofen play around with kaggle data, Kaggle is a data science competition platform where companies will start competitions to solve their problems, there's frequently cash prizes for the top guys.
NFL has been posting alot of competitions lately mainly regarding players safety. The most recent one though was about predicting rush success based on the RBs speed, direction, game state and many other variables. I wouldnt be surprised if the NFL came to the conclusion that the best way to protect their QBs would be by encouraging teams to run the ball more. I makes total sense IMO, but as stated earlier, this is pure speculation on my part.