A couple of things.But without looking at the stats, my sense is that the defense didn't make big-time plays down the stretch of the season. It felt like the pass rush dropped off significantly from the first half of the season. This was my perception before McVay mentioned lacking closers. Once he and Snead mentioned that, I agree that that was the biggest issue at the end of the season.
As far as covering JSN, the pass rush affects the coverage. Having a closer will help keep him in check.
The Rams played 20 games last year.
In the first 10 they had 27 sacks and 39 TFL
In the last 10 they had 25 sacks and 42 TFL
They played Seattle 3 times and went 1-2.
In the 2 losses they had 7 sacks, 11 TFL
In the 1 win they had 0 sacks, 0 TFL
Certainly pass rush doesn't mean sacks though many fans equate it that way. In the first Seattle game they hounded and confused Darnold into 4 picks.
One question though. Did McVay actually say, the reason we lost last year was because we didn't have a closer. Lost what? 6 times or lost the CCG?
I'll stick to my opinion a "closer" or lack thereof was only 1 of a possible half dozen reasons we lost games.