Just healthy competition for places in my opinion. Let’s be honest, we’d be shitting it if Ramsdale got injured. Potentially season ending - because Ramsdale is so important to our style of play. Talk about mistakes are a red herring, this is absolutely about continuity and ability on the ball.
Keepers usually don’t get rotated because there’s a big gap in quality. Otherwise I don’t see why not. In theory there’s 60 odd games a season potentially, you could conservatively play a second choice for 25 games.
It’s also worth remembering when considering competition for places that the CL is being expanded next season. We will be playing twice a week - in major competitions.