Agreed, but I always think this point is taken too far in the direction of devaluing those great volleyers' skills (not that you are necessarily doing that).
Not many people during the aforementioned eras could volley like Mac and Edberg and Cash against even the top players in their eras like Lendl, Becker, Connors, Borg, etc. Volleying against top players may be harder today, but it was never exactly easy and it's not like just anybody could rush the net against Lendl or Connors (for example) and enjoy instant success. Even if it's more difficult today, it was difficult then too.
I often hear that these great volleyers would be toast today because of the consistent pace and the dipping nature of highly spinny strokes. But, I've rewatched a lot of McEnroe and especially Edberg matches and they took a lot of vollyey below the net, many down low. Not every volley was hit in no-doubt territory above the net. And, there were players who hit with considerable (if less than today) spin and dipped shots, and players who blasted the ball.
I'm not arguing with you're general point - today's volleyers, overall, have it much tougher and would find consistent success much harder to come by.
But, I still think volleying could and should be an important part of a player's arsenal and the downturn in volleyers' collective fortunes isn't due only to more consistent spin and pace coming off players' rackets. Also, as has been noted endlessly, the slow down in surfaces. Put Boris Becker or Sampras against a Nadal-type on slick grass or speedy indoor carpet and, and I'd think they'd do okay. yes, the Nadal-types would get more passing shots past them, get more good returns back, but I hardly think the balance would be so immutably shifted that Becker and Sampras could never win.
Also, I just don't think volleying as a gamestyle is really tought that much any more, so there are few elite volleyers. Clearly, every tennis player learns to volley (the actual stroke), but fewer really learn how to incorporate it into their game other than when hitting a huge approach followed by an easy knock-off volley or when forced into net against their will and having to make a deperation volley.
It's a skill that takes a while to develop and without repeated "reps" in the form of repeated trips to the net, a player will never develop into a great volleyer. Beyond the actual volley, a player has to learn and develop anticipation at the net, passing patterns of his opponents, lateral and forward movement at the net, percentages of where to lean on which approach shots.
But, no player does repeated "reps" any more in match play, thus volleying suffers. This is only exacerbated by junior tennis, where all players, naturally, want to be successful from an early age, and being an elite volleyer rally takes physical maturity. High-spin groundstrokes are the norm and a 5 foot tall 12-year old boy can more quickly develop those groundstrokes than an impressive volleying game. The 5-foot tall 12-year-old on the other side of the net who isn't even tall enough to have a good serve to set him up for volleying is going to usually lose the battle if he comes to net. So, everyone is sort of forced to play the same game, the same game we see on the ATP tour, at a much lower level, of course.
A friend of mine went to Macci's academy for one year in 1990 or 1991 as a teenager.. They were taught "Rush and Crush" tennis. Serve big, hit big and finish the point at net. Everything was move forward becuase that is what worked at the time. This is before the slowing down of the balls, courts and the take over of Babalot and Luxilon. They were taught what won. Now baseline play wins.