It's simply unrealistic at the NHL level, against top-10 teams, to expect players to routinely come back from constant lay-ups into their own net. I don't know if any of the teams that won the Stanley Cup in the last 15 years would be cup champions if they had to overcome a 1-0 deficit before the game starts in most of the games they played. The parity in the NHL is just too great to expect a team to get over that and still be elite. It's not realistic.
What's worse is that even when Koskinen is in the zone, he's liable to let in a brutal goal. NHL starters need to make the saves that Koskinen makes when he's "on," which people seem to sometimes think means he's "stealing a game." But the reality is, NHL starters make those saves; that's why they're starting in the NHL. Koskinen is not doing anything above and beyond what the team should expect of a starter when he "bounces back" from his routine muffin. The big difference between an actual NHL starter and Koskinen is that the starter makes all the saves Koskinen makes and lets in a squeaker every once in a while, whereas Koskinen lets in that muffin almost every game.
This is who is and always has been. The team has had to score 4+ goals to win more nights than not (including the nights they lose), and a big part of that is goaltending.
Look, I agree that last nights 1st goal was a terrible goal. I also agree that given the body of work Koskinen has put forward on this team he is not an NHL starter. That's not in debate. We need to move on from him.
That said, he has been playing quite well. You say ~ treading water. I disagree, I think it's been a bit more than that, but again... I'm not here to defend Koskinen.
I'm simply making two points:
1) We have complained about 1st shot/early goal-itis as an excuse for at least 4-5 goalies. Scrivens, Talbot, Koskinen, even Smith. First, 2nd, 3rd shots on goal go in a lot against most teams in the league. It is not an excuse to pack it in when you get scored on early. That's a losers mentality.
2) The goalie is just like any other player. He's responsible for his own mental state of mind and to compensate for mistakes made by his teammates. That's hockey.
When a goalie blows it on a play, (as Koskinen did) he's responsible to not blow it on the next play and get his head in the game. Koskinen's mental lapse on that play (it was a mental lapse, he's clearly capable of making that save... that's why we think it was a weak goal) is just that. A mental lapse.
It's no different than a defenseman taking his eye off the puck and having it bounce his stick at the blue line leading to a breakaway against. Zero difference except that the goalie will have a chance to bail out the defenseman, and usually a goalie gets no such luxury.
Given that higher-stakes consequence, it is completely unreasonable to ALSO make the goalie responsible for his teammates' ability to recover from a goal on subsequent plays. That is on them to shake it off.
IF you think it is ok for a whole teams' worth of players to not overcome that weak goal/injury to the psyche, then it would follow that the goalie doesn't need to worry about stopping a breakaway when player X makes the same repeat mistake at the blue line. Teams that think that one guy's momentary lapse in concentration is any worse than the next guy's lapse in concentration are not TEAMS.
Now... that said, players that consistently make errors in judgement and mental mistakes are not the types of players that should be holding down primary jobs at the NHL level. Physical talent can of course overcome some of those mental errors, but ultimately the balance has to be in favor of positive performance/winning. With Koskinen it is not (though I would argue his deficit is at least as much physical as it is mental... his reflexes do him no favors).