well lets have a standard league bat for sake of consistency?
Not quite the same thing though. It's a ball sport, the league presides over the ball, whereas the players preside over their own equipment which can, and will vary.
When i played football no-one was telling me i had to wear standard issue boots, or goalie gloves. And particularly with gloves, some were much better than others. Being given a new set for free every few weeks by my sponsor meant i never had to worry about it, but i certainly would have purchased better ones myself if i had to, just for the added advantage over my opponents, however small an advantage that was.
Same with Tennis, the ball is the same, but rackets are all different. Hockey, baseball, even one of my niche favourites snooker. The ball is largely the same, certainly each tournament/competition/league/national board/etc will have a standard issue ball, so if touring the globe on the world tennis tour you will encounter some variation but not match to match when playing in games under the banner of the same board. The players job is to find the best equipment he can to enable him to play to his best at the chosen sport. My friend hates my snooker cue for example, but i love it. His is much cheaper than mine (the perk of having an account with GM who also have Powerglide cues under the same umbrella) but i would happily use his. I still think mine is better. If we were forced to use a standard cue i'm sure neither of us would be happy. We accept the standard ball though even if they are too big for the pockets
I accept cricket is somewhat different as there is much more variation in cricket balls than tennis or hockey etc. I think some of your previous points are what we need to concentrate on. Covered wickets, smaller boundaries etc are all aiding the batsmen. Poor balls issued by leagues don't help bowlers, but if you collectively put enough pressure on a league board they will change.
I just don't feel its as simple as 'modern bigger bats hit the ball further' - i mis-timed my first ever six 11 years ago using a harrow that was basically a twig.