Your homogeneous theory is a flaw because Borg was consistently winning RG and Wimbledon back-to-back. You can argue homgeneous sufaces is even tougher to win because the entire field can compete at high level, unlike specialists where there's a few players that are contestant to win on certain surfaces(lol). All rounds are tough today because they are great baseliners on all surfaces. I suppose you believe it would be better for Sampras had every surfaces played like Plexi(
). And even specialists in the 90s weren't always making the final(eg Courier who isn't a grass specialist yet made the 1993 Wimbledon final). That's proved there's holes in the 90s where the window of oppotunities for non-great players can win a slam. Unlike today, all surfaces are heavily guarded. There's no weak players sneaking in to grab the slam.
Don't forget even though the surfaces are more homogeneous today, the players still have their best and worst surfaces. For Nadal is clay and hc, Roddick is hc and clay, Roger is grass and clay, etc....