There were plenty of good keepers around in Schmeichel's time. During Schmeichel's time at United the likes of Seamen, Oliver Khan, VDS, Zubbizerretta, Zenga, Barthez and Toldo were all playing for top clubs and his end of United there were the likes of Buffon and Casillas just coming through. There was a time in the 90's when Ac Milan's keeper Rossi who played in three straight Champions league finals could not get a cap for Italy.
I look around the standard of keepers today and there's not a group of keepers around as good as that.
Back to comparison with Schmeichel. For me, De Gea has to do at the highest level to be even in the debate. Schmeichel was a keeper making saves which won European cups, Premiership titles and European championships when the pressure was a lot higher in the biggest games. For me, that worth far more in terms of legacy than making a lot of saves for a team challenging for 4th place. I've seen Schmeichel makes 8-10 saves a game against games against title rivals which have made the difference in winning league titles, save penalties in massive season defining games and make key saves which turned key games against the best the world had to offer.