Watching the Oilers last season there’s no way anyone could agree with this. Two different goaltenders, same defensive scheme, two vastly different results depending who was in net. No defensive scheme is going to fix soft goals that a goalie shouldn’t let in.
Koskinen gets wore down with over usage and he can't play multiple games per week for weeks on end. And when he gets wore down it takes a long time for him to recover. I won't get detailed and I certainly haven't prepared any charts.
I've seen Koskinen excelling as much as I've seen the player broken and defeated. Then there was a bunch that fits between these two extremes. Playing a significant number of games over a season is probably the number one requirement for being a starting goaltender and it's clear he is not that.
The old guy, Smith, is capable of playing more games than Koskinen but he shouldn't be expected to play 5 game stretches either. Tippett will have to employ a strict regimen of game management for his goaltenders, this includes the possibility of Stalock being named full-time backup or even the contingency of the dreaded three headed monster. It should be noted that they can waive Koskinen as many times as they want without him actually leaving the Oilers, at home or on the road, and the probability of him getting claimed is remote.
The greatest goaltenders I have ever seen play the game, in the modern era, all had one thing in common regardless of age or style played, the teams in front of them played an unwavering defensive structure with the "bulk of shots" taken from identifiable, and more importantly, "predictable areas" on the ice where the shooters were effectively corralled by the defensive system employed with grade A scoring chances from the slot coming at a premium.