Like I said on the last page, the skill gap difference between professional goalies is incredibly small. That's why you see examples of beer league adult goalies going into NHL games and actually being able to make some stops. The skill gap difference between goalies is substantially smaller than it is between skaters. What makes the best goalies the best goalies is that they can do it every night. That's what DeSmith doesn't have, he can do it in spurts (like he did yesterday) but he can't consistently maintain that level.
There are truly elite goalies that take the skill level to a new high, but very few goalies fall in that category. You have maybe 5 goalies in the NHL right now who fall into that group (Vasilevskiy, Sorokin, Shesterkin and Hellebuyck are the current ones IMO). But pretty much every other goalie falls in similar skill ranges, it's just the starters have consistency levels that the backups don't have. It's why you regularly see goalies go from having a great season as a starter to being a shitty backup. It's all just a law of averages with most goalies, if they're playing at unsustainable levels for a season, they'll likely come crashing back down in the next season.
I firmly believe you could put any high level goalie (meaning anything from juniors up) in the NHL and they can get a shutout in a NHL game. It's just like when Alexander Pechurskiy came into a game for the Penguins in 2009-2010 when he was an okay WHL goalie and stopped 12 of 13 shots. With that being said, if you want a goalie who will be giving you a .925 save% every single game, your pretty much only option is to internally develop them. Maybe you can get one by paying them over $10 million in free agency, but it will almost definitely need to come from internal development.
The TLDR of this is that DeSmith isn't a problem as a backup, his issue is that his inconsistencies are getting exposed because he's playing too much due to Jarry's issues. Any other backup goalie you could get instead would have the same inconsistency issues. They could totally be performing better than DeSmith, but that good performance is also going to come with an inevitable shitty performance.
You may think I have really dumb takes on skaters, coaches, GMs and whatnot, but I'm 100% confident in my goalie takes. I played goalie my entire life and understand what goes into the position. You can't apply skater standards of "if a player is playing bad, it means he's not good" standard with goalies.