I very strongly disagree with this sentiment. Defense is the most important position in professional hockey by a long shot. A mediocre goalie is the easiest thing in pro hockey to cover for, even in the playoffs. Chicago won with Niemi. Philly got the the final with Lieghton. Ottawa regularly went deep with Anderson. Murray has 2 Stanley Cups. Elite teams don't need great goalies because they possess the puck on the other end of ice for the vast majority of the game. That starts with a great defense. The offense putting in pucks consistently makes goalie fairly inconsequential. That doesn't mean you can have Scott Foster as your starter, but having someone in the bottom half of the league in goaltending ability doesn't really matter if the team around him is Cup quality.
Ottawa got out of the first round twice with Anderson. One of those years they lost 4-1 in the second round. In the other, they got to the Conference Final with Anderson. He posted a .922 and played pretty damn well based on my eye test. He also posted a .926 in the regular season that year (through 40 starts) and was widely considered a top 10 goalie that season despite his starts being limited due to his leave to be with his wife.
Murray has a career .923 playoff SV%. He posted a .923 in his first Cup run and a .937 in the next. He was mediocre last year and lo and behold, the Pens got bounced in the 2nd round.
In 2011, Thomas won the Conn Smythe with a .940 SV%. He won the Vezina.
In 2012, Quick won the Conn Smythe with a .946 SV%. He was 2nd in Vezina voting that year.
In 2013, Crawford posted a .932 in the playoffs in his first Cup run.
In 2014, Quick posted a mediocre .911 SV%. He was abysmal in their first 3 games against SJ, but then became a superhero for the reverse sweep. He was also brutal for a 3 game stretch in round two before flipping the switch again. He was decidedly mediocre overall, but was incredible slightly more often than he was terrible, which allowed the Kings to win 3 straight 7 game series because he was great slightly more often than not.
In 2015, Crawford posted a .924 SV%.
Murray posted the .923 and .937 totals I mentioned above in 2016 and 2017.
Holtby posted a .922 last year.
I don't see any teams covering for below average goalies in there. 3 of those 5 goalies have a Vezina trophy and Crawford has finished as high as 5th in the voting and is unquestionably a top 10 NHL goalie. Murray struggled in his first year without a safety net, but was unquestionably fantastic in their two playoff runs.
In terms of losing goalies, the last 3 years have seen Rinne, Fleury, and Jones all play fantastic in the first 3 rounds of the playoffs. But they all came down to Earth in the Final. We're probably talking about different Cup winners if those 3 had sustained their great play instead of forcing their teams to cover for mediocre goaltending in the Final.
I completely disagree with the idea that it is easy to cover for a mediocre goalie. If you want to win a Cup with that formula, that goalie needs to play like a top 10 goalie in the playoffs (even if he can't sustain it all season). Otherwise, you better have what is by far the best roster assembled in the cap era. That 2010 Hawks lineup was just hilariously absurd and that was 9 years ago. Since then, we've seen 1 out of 8 Cup winning goalies post a sub-.920 playoff SV% and that 1 exception had a weird habit of single handedly winning and losing games instead of just being 'meh' throughout.