Bortles is not an accurate passer. And when looking at how that impacts his "fit" in this system, the WCO doesn't historically require the strongest arm, but accuracy (to include being able to throw with anticipation) is paramount. That allows for RAC which is crucial.

Where McVay's offense is different than the old WCO is it also stretches the field vertically. He takes a lot more deep shots and so to do what the Rams currently do requires a passer who is not only accurate but who has enough arm to place the ball down field.

Enter Goff. Who meets all those requirements. Goff is a big part of the reason McVay's offense is so good with the Rams. A lesser QB would still give us a good offense under his superb coaching and scheming, but not what we are seeing. An example of what I'm talking about is the Skins' offense in his OC years...

2013 (Kyle Shanahan was OC) 23rd in points.
2014 (Sean McVay) 26th in points.
2015 (Sean McVay) 10th in points.
2016 (Sean McVay) 12th in points.

Commensurate with the rise in points above was the rise of Cousins. Who was accurate but who lacked the ability to stretch the field vertically like the Rams do now. His Rams offenses...

2017 Rams 1st in points (478!!!).
2018 Rams 2nd in points (527!!!).

Goff, again, is a huge part of the reason this offense is so different (enormous usage of wideouts with very little TE usage like he had in Washington) and why it is so potent.

McVay would never consider that no matter how good Bortles might look in preseason. Because Bortles is not only inaccurate but he's also quite streaky which is a combination that can drive a guy who runs a surgical passing game like McVay crazy.