Again, much of what you say I agree with. Increasing the player pool in 1920 to black and Latino players you would have had commensurate numbers.
Yet, bigger, stronger, faster doesn't necessarily mean higher averages, and I disagree that today's players are smarter. Steroids make you bigger, stronger and faster, yet a no expert will tell you it increases averages. A great batting eye is a great batting eye. Great hand/eye coordination transcends eras. Bigger and stronger will make the ball go further, no doubt. Shorter fences increases the number of homers, no doubt, and averages are hurt by better fielders and gloves. But in the dead ball days you had to choke up and get the single, play the hit and run, steal bases, play a more cerebral game, because the ball didn't jump off the bat as today. With the larger parks of Sisler's era, yeah, you had no choice but to play the short game and spray the ball around the park. Players today don't even try to hit for average. Even the guy batting ninth swings from his heels. Are you saying that if a good young hitter took the attitude of a Sisler: Choke up, use the lines, use the gaps, step in front of pitches, chop the ball, bunt more, instead of overswinging, that he wouldn't approach the same numbers? For a guy like Sisler, it wouldn't matter how many different pitchers or pitching angles he'd see in one game. He's not going to try to beat you with the long ball, he's just going to make contact. Yes, I think with Sisler's approach and today's training techniques, if given a chance to play, he'd have a healthy average approaching his lifetime average.
Yes, today's pitchers have good velocity. I would argue about the variety of pitches. The use of grease and other additives I think was more prevalent back in those days, although the use of the slider has definitely increase during this era of ball. And yes, today's analytics and tracking has revolutionize sports. However, the strike zone shrinking has as much to do with today's HR's and good hitting as much as analytics and lowering the mound.