I'd guess the loss of the slip-stroke as a technique was natural as players transitioned from playing mainly 14.1 to 9 ball. Those 14.1 players tended to grip the cue much further forward than today's players. All the short, precision shots required in 14.1, just didn't need as much cue power. On the odd shot where a player would need a little more oomph, they would naturally slide their grip hand back.
Fast forward to playing mainly 9 ball, and players realized it made sense to move the grip hand back. I do think here in the past 10 years or so, players have greatly exaggerated their "need" for more cue as more and more players added cue extensions. I think a lot of players, even really good ones, misunderstand where cue power comes from. Just putting your grip hand further and further back, doesn't necessarily give you more power. It may actually be the opposite. It's all a bit counter-intuitive.