T20 did not ruin test batting
The complaint arrives every summer, usually from somebody watching a collapse. It is wrong, and the reason is more interesting than the complaint.
A top order gets rolled for not very many, somebody plays a shot they would not have played in 1994, and within the hour the same line arrives. They cannot bat time any more. It is the franchise cricket. They have forgotten how.
It is a comfortable story and it does not survive much contact with what is actually happening in the middle.
The shots did not come from where people think
Batters play more strokes now, and more of them are premeditated. That much is true. What does not follow is that the short game taught them to be reckless, because the same players who came up in the short game also defend with better balance and read length faster than anybody did thirty years ago.
Being able to hit a ball over cover off a length is a skill added to a batter. It does not remove the forward defensive from their hands. Nobody forgets how to leave.
The thing that genuinely changed
Volume. A player in their twenties now spends most of the year in white ball cricket because that is where the calendar and the money are, and a skill you use twice a year gets rusty. It is not a moral failure and it is not a technique problem. It is a diary problem.
The other real change is the ball and the surfaces, and that argument at least has something in it. A pitch prepared for a result and a ball that swings for a session and a half will make any generation look like it cannot bat.
Why the complaint keeps coming back
Because it is easy, and because collapses are memorable in a way that patient hundreds are not. Nobody sits down at the end of a long summer and says, actually, that was a decent series for the top order. They remember the eleven overs where four wickets went down.
Cricket has been declaring itself ruined since before any of us were born. Overarm bowling ruined it. One day cricket ruined it. Helmets, covered wickets, coloured clothing, the lot. The game keeps not being ruined, which after all this time is fairly good evidence.
What we would actually argue about
If you want a real complaint about modern batting, try this one. The gap between the best players and the tenth best has grown, because the ones with the schedule and the coaching keep improving while everybody else plays a format lottery. That is worth being annoyed about. It has very little to do with anybody hitting a six.
The short version
The short game gave batters more shots and less time to practise the old ones. Blame the calendar and leave the batting alone.