1st and 10 with 1:33 left we run the ball. Seattle has three timeouts but does NOT call one. Gain of 5 yards.
2nd and 5 is snapped with 1:13 left - note here that the Rams could have bled the clock down to roughly 50 seconds since Seattle did not call timeout. From here, the odds of picking up 5 yards on the ground in two plays are decent. But for the sake of your argument, lets say we run twice for no gain and Seattle calls timeouts. In this hypothetical we'll assume both plays only take 4 seconds off the clock:
2nd and 5 is snapped at 50 seconds, 3rd and 5 is snapped at 46 seconds. After the 3rd and 5 play, Seattle calls timeout with 42 seconds remaining. The Rams bring on the punt team. We know that punt took 9 seconds, so Seattle calls fair catch at their only 26 yard line with 33 seconds remaining and one timeout.
From there, they MIGHT try to get a FG - but they have a defensive minded HC, and a QB who led the league in turnovers. Its equally possible that they just run the ball, and its also likely the entire defensive sequence plays out differently. Best case for Seattle they get 3, most likely is they don't score.
And again - that's after a first down run went for 5 yards and then assuming the other two don't pick up the first down, which given our success rate in general running the ball, seems likely.
The flow of the game dictated being conservative there. We had no business going into halftime with a lead based on how that game started, yet that's what was about to happen. It was a possession that Seattle never should have had. So even with the muffed punt gift TD, we likely win if we just run the ball here, which is why the OP says this drive lost us the game.
Of course, Seattle would have had the ball at some point down by a score so to your point about how quickly they moved through our defense, there is no guarantee that this alone does it. But it was still a huge, huge mistake by McVay.
