So the team loses because the head coach gets dumber as the season wears on? Then he's smart again at the start of the next season? I think there must be a better explanation for this pattern. It might not even be the same reason each year.
The best players peak at the right time, late in the season, when the stakes are higher, and during the playoffs, under pressure of elimination. That also applies to the coaching staff.
Of course, reaching the playoffs requires a good regular season, but regular season records, and championships are easily forgotten.
On a macro level, the same principles also apply to each game. Playing well in the first half means nothing if there is a second half let down or collapse. The best coaches have the ability to read both their team, and the opponent, make halftime adjustments and win games.
And it's not just football. In baseball, a pitcher may be successful at getting outs the first, or second time through the opposing order, but if they don't adjust, and throw the same pitches again later in the game, the hitters are going to figure them out and get hits. Pitchers who are untouchable the first time a team faces them can lose their edge in each subsequent meeting, if they don't adapt. Good stuff isn't unbeatable, and can only go so far.
Of course, a lot of factors go into wins and losses, and the coaches aren't the ones who have to make the plays, but they provide the guidance and leadership to put their players in position to perform.
Many a talented team has been disappointing, as can be said about the Cardinals, and the coaches have to bear part of the blame.
Conversely, the Rams also started well, began to falter a bit during the season, but then got stronger during crunch time, thoroughly handling the Cards, beating the Bucs, and then finally the 49ers. McVay made adjustments and put together a game plan to run against them, hold onto the ball, which wore down the 49er's strength, their defense, and D-line and linebackers in particular. Just enough for the Rams offense to take the lead late in the game.
The NFC West is the strongest in the NFC, if not the whole league. Despite the down year, Carroll's record speaks for itself. McVay and Shanahan may not be elite, yet (or perhaps ever), but still very good and in the upper echelon in the league. Until he proves otherwise, KK is the odd man out.