How has Robles saved us from a relegation battle? We're worse off now than we were when he took over! Since he's been our starter, we have twelve points from eleven matches. Like I said, he's been good and it's not his fault, but the goalkeeping switch just hasn't helped.
Look, De Gea is one of those handful of elite keepers who actually make a significant difference. He's definitely won points for United on his own. If you think Schmeichel's on that same level, though, I don't think you've watched many Leicester games. He hasn't been bad or anything, but swap him out with Gomes or Cech or Adrián and I think Leicester are still winning the league.
Take a look at some of the clubs who have changed goalkeepers during the season and compare their results with their "starter" in goal and their "backup". There's hardly any difference. Just like there's been no improvement at Everton since Joel took over. The only team that has seen any sort of improvement after benching their starter this season has been Sunderland, and that probably has more to do with Allardyce taking over the club at the same time.