Is this one of those things where you're taking all-time great exceptions to rules and trying to apply them to the league as a whole? How many hundreds and hundreds of defensemen played during the span of those players' careers...?
There weren't hundreds and hundreds of defensemen whose careers spanned those of MacInnis, Chelios and Stevens that were of a high stature.
I do think the high-end talent pool was bigger in the 1990s than in the 1980s because of the European influx, and it's quite obvious goaltending was better in the 1990s, but the idea that the NHL suddenly became much better in terms of quality of play after the Gretzky era is overblown. It's not like there was a war that many of the best Canadians players were fighting in during the 1980s. The League had a vast pool of Canadian talent to draw from and there were already Americans and Europeans in the NHL.
The
style of play changed in the 1990s, but that doesn't necessarily mean defensemen suddenly became much better. Some of the best defensemen of the 1990s were accomplished in the 1980s. Goaltenders were supported better and their equipment became larger, but while goaltending and defensive improved, creativity declined.
Fringe forwards have become better over time and there is less of a gap between the elite and non-elite than there used to be, but there are numerous examples of high-end players who played in the 1980s continuing to play at a high level into the 1990s and/or beyond (including Gretzky himself).