These are all active goaltenders under post-lockout rules. The youngest goalie on this list was Age 24 when they had completed their 8th playoff start.
Had I pulled up older goalies under different rules, the statistics are insane in what goalies did with teams being allowed to hook, hold, interfere, etc.
And guess how many goalies on the list above had won a series in their 1st 8 starts? This is the standard you are applying to Allen. 8 starts, the starter for 1 playoff series. One series loss. No way he can win in the playoffs, ever.
Husso won't be here next season. At the earliest, he's a backup in 2018-19 assuming he develops faster than normal. Allen's numbers warrant the contract he received based on what other goalies are being paid and Allen's Age. There are quite a few performing quite worse being paid significantly more.
With the contract ending in 20-21, the salary drops to 4M that season - barring strange occurrences, that's when Husso likely becomes a full-time part of the Blues roster. Allen is being paid as a starter for what is likely the time it will take for Husso to be ready.
Husso - 21 right now.
Allen played 15 games with the Blues when he was 22 as an emergency recall.
He didn't officially take over as backup until he was 24.
So in 3 years Husso will be 24 - oh, that's the 20/21 season - the last year of Allen's contract, when Husso should be ready to be a full-time part of the Blues. And this is normal for most goaltenders who reach the NHL and stay - right around age 24. There are exceptions in terms of goalies being ready sooner, but not many of them.
But go ahead and berate the contract without considering all context - as you've done with the entire Elliott situation.