It's also too clear cut to be the most likely scenario.
Thing to keep in mind is Goff is not McVay's guy but he is the rest of the brain-trust's guy. Any replacing of Goff as the starter due to a performance failure will effectively mean the end of the Goff era in LA and that will be an organizational decision, not solely McVay's.
Because there is no coming back from that and the investment in him is so massive, he's going to have to consistently be REALLY bad for a long time before they'd push that eject button. I think we're talking worse or as bad as last season bad and I don't think that's likely to happen.
The more likely scenario is that Goff's performance this year is uneven but overall neither bad enough to persuade the organization to move on from him nor good enough to silence critics and doubters. He'll start all season as long as he stays healthy and the debate about whether he's going to be good enough will carry on into the off season. And they probably won't draft a QB high, even if one of the top rookies is there for them, because they think they need to give him another year.
Ultimately, if he's not going to be the franchise QB he was supposed to be, it would be better if we get your scenario and he clearly fails fast and they can start moving on. But that kind of clarity is rarely delivered so swiftly in this kind of situation. Absent catastrophic failure, he'll be the Rams QB after this season and we'll all be back here next season having the same or similar debates about him.