Quality balls is a good point, and I know for some time there has been a rising concern about the increased use of Speedarm throwers in training. Link for those who aren't sure what I mean.
It might be good for an older coach who's not as mobile or may have dodgy shoulders to be able to throw some balls down to his young charge, but it's just not going to replicate the real thing of a bowler coming down who's balls can vary from brilliant to wayward and everything in between, and the batsmen needing to be prepared for all possibilities.
There's no artificial method of launching a cricket ball down a wicket that gets even close to facing a bowler. I reckon batsmen are probably facing significantly less quality bowling these days for sure. I'm not sure what the solution to it is. More net bowlers? Less gym more bowling? I don't know.