As a counter to your argument the senators over the last ten seasons have been in large part influenced by goaltending performance. We usually make the playoffs when our goalies are having strong seasons and must them if they don't . But if we look through a number of our previous coaches I think it would be difficult to argue that the majority were good or competent.
So if you argue that Boucher is an adequate coach and he was had the unfortunate luck of bad goaltending couldn't you make the same claim for our previous coaches?
If anything I think your position supports a different argument than the one you are making. From what you said it suggests that goaltending quality may be more important than coaching quality, at least in the short run.
Goaltending is a vital position and we have seen mediocre rosters perform well with elite goaltending such as in Montreal. We have also seen arguably average goaltending offset by an elite defence in Chicago during their cup runs and Anaheim in 2007.
As for coaching, I am not sure which data we could look at to make a proper evaluation. There is likely some minimum performance by goaltenders to make the playoffs and anything below that would make it impossible for a coach to be successful. There must be a handful of teams with average goaltending that were playoff calibre. If we identified them we could maybe assess what the coach did and how to evaluate a coach under those conditions.
I agree with you that there is some minimum performance required by goaltending in order to make the playoffs. year 2 for Boucher both here and TB he didn't get numbers anywhere close to what the minimum performance would be. on the one hand it is valid to look at his year 2s and state they sucked. they clearly did. it's also valid to point at goaltending and simply claim you cannot be successful with that level of goaltending and you can't.
in 2017/18 the league average save percentage was 912 and ours was 899.
in 2011/12 the league average save percentage was 914 and TB's was 893.
no coach is going to survive let alone prosper getting that kind of goaltending.
if you look at it over a season, the gap between us and average goaltending would be around 35 goals pushing 45 goals with a bit better than average.
you end up chasing games you shouldn't. you end up tied in games you should be leading. losing games in OT you should have won in regulation. not getting a point in games you should have taken to overtime.
we gave up 12 empty net goals last year. without examining stats game by game, roughly half of those games we likely gave up an extra goal. that's 6 games that should have gone to OT and if you win half of those that's 9 points.
but what's worse than anything statistically is the players know the goaltending is weak.