Quoting: The_Ultimate_Pielord
The point of having one good goalie vs 2 mediocre goalies is that come playoff time rest isn't a major issue, so you only use one goalie and the better that goalie is the better the team is. The ideal goaltending situation is something along the lines of Arizona or Anaheim, where you have an elite goaltender and a good enough backup to only play them 45 times a year (my argument would work better if either of those teams could make the playoffs, but Carlyle and the Arizona injury bug have decided that I can't have nice things.)
Let's say we have 2 teams. One team has one goalie worth one win per 10 games (very very good) and one goalie worth -0.5 wins every ten games (less good). That comes to +0.5 wins per 10 games from goaltending with an even workload over the regular season for maximum rest, about +4 wins on the year. The other team has 2 goalies each worth 0.3 wins per 10 games, for 0.6 wins per ten games from goaltending, or 4.8 per year. Which team would you rather be?
The answer is the 1st one, for 2 reasons. For one, you can adjust the usage to make the 1st one get more wins. If you change it to a 50-30 split (using round number for easier mathing), suddenly that's +4.5 wins, or almost even. At a 60-20 split you've got +5 wins, with a 0.2 win cushion to account for diminished performance due to lack of rest. The coach can ride a #1 goalie hard to get a bad team into the playoffs, and can give some rest if the team is good enough to survive it, while a team without one is left in a trickier spot, even with a goalie tandem that's better overall.
The 2nd answer is playoffs. Teams pretty much never use their backups in the playoffs, even when the dropoff is pretty small, and rest generally isn't a factor in the playoffs thanks to the lack of travel. With that in mind, over a 7-game series, team #1 will get 0.7 wins from goaltending (1 win times 7/10ths of a ten game stretch), while team 2 will get 0.21 (3 tenths times 7 tenths) in that series. Half a win might not sound like a ton, but in a tight series it means everything.
Investing a ton in a number one goalie isn't a great idea with how inconsistent and difficult to project goalies are, but there's no denying you'd rather have a #1 than a platoon.
Okay first, a .3 goalie and another .3 goalie don't get you to a .6 goalie, every game would be played at a .3 Wins Per Ten pace so obviously you would be at .3 wins per ten for all 82 games. But this theoretical stat game is a zero ends game because i can give you stats that say one thing you can refute with a different one that says another. SV% doesn't account for quality, Wins above expected doesn't give enough credit to goalies with defensively solid teams, and doesn't take in to account quality, GAA is literally just a team stat, WAR is OK but oversimplifies whats going on and punishes goalies in good defensive systems. Plus all goalies are unicorns, defining them as if they will act consistently year round is disingenuous.
But on another note, the Ducks are proof that over work is definitely a thing, Gibson is the best goalie in hockey period in my opinion, and early in the year the Ducks ride him to being competitive in the early playoff race. But after January his stats begin to really fall off because its not just a volume of games played it is a volume of shots he faces and a very uniquely high volume of quality chances to score against him. Basically Gibson faces a constant barrage of rubber to the face every night for 60+ nights unimpeded by his offensively focused defense. Under this system the Ducks backup gets absolutely torched in the fire that only Gibson is able to handle. Gibson under the Islanders system (Assuming his play remains just as good which isn't true but theoretically speaking) his numbers would be godlike. He's not a better goalie in New York, he is very much a product of environment. The Islanders have a very competent defense and don't let quality chances through often at all. Thus, the reason that you see Robin Lehner, Thomas Griess, and now Varlomov are all putting up numbers that are equal too Gibson's (SV% wise, GAA, GPG, Wins). All flawed stats in their own right but so is Wins above expected. These goalies aren't better than Gibson in any world but their environment makes them play at a level that is equal too a Gibson. Gibson would put up a .935 sv% in NY, Lehner put up a .931% Griess put up a .927%, basically, by putting their money and focus on building up a strong defensive system they have given themselves a goalie that is equal to what Gibson would do every night. This is accomplished by understanding the mechanics of being a goaltender in the NHL. Most guys that are good in the NHL can stop the shots a NYI goaltender faces on a nightly basis, no goalie on the planet not named Gibson or Jesus himself can put up the performances Gibson puts out on a nightly basis in Anaheim.
What I just described to you is the future of hockey, by letting Lehner walk in free agency the Isles demonstrated they understand what they've done, they have created a system where all they need is a goalie that is positionally sound and they will preform the same as if they had Carey Price. Thus they can spend less on a goalie tandem than anyone else have it play better than anyone else, and invest the extra Draft capital and extra money capital on maintaining a good defensive core, and to pay Barry Trotz whatever the hell he wants to keep running it.
Im not arguing simply that two decent goalies are better than one Amazing one, cause thats not true, im arguing that understanding the goalie position and practically eliminating the high danger scoring chances by bolstering your defense will produce a tandem of goalies that, if they are positionally sound, will both play at a level equivalent to a Gibson or a Price in their systems which often leave them out to dry.
This is better for playoffs because your season can't be jack knifed by one player being injured, if one of your goalies gets cold and starts letting in weak pucks you have another that will work equally well, and you have more money and draft picks that you spent in making your defense better so your team can theoretically be better in the process. This is economics merging with hockey to make our game more efficient and smarter, this is the future of the game.