It's baffling because the evidence isn't there to support it at all.Just so you know, I have an opinion and so do you. 'Baffling' probably isn't necessary.
Explain to me how you know T20 'never seemed' to have an effect? Because as I recall, T20 started becoming a real thing both domestically around the world and T20Is around 2010 (little earlier). How could you rule that out being part of the reason Test players aren't as effective? ODIs and T20s, I think you could appreciate, differ in terms of their jump to Test cricket. So that transition doesn't stand up as similarly disruptive.
Jarrod Kimber is a bloke with an opinion, too - just FYI. Smart guy, by the looks.
And you're saying England is a much harder place to bat than New Zealand? So things have changed since Cook, Strauss, Pietersen, Trott, Bell, Collingwood and Prior averaged 40 playing at least half their Tests, if not more at home? That's one generation of players and I just named 7 again who averaged over 40.
It's all good, there's no exact reason for the dip in the 2010s. You take your view, I'll take mine.
Because there hasn't been a noticeable change that can be attributed to T20s or ODIs at all. When T20s became a thing we still had flat pitches good for batting, and even now when things get flat we can see scores of 400+ by Test sides with both all format players and Test specialists cashing in, which wouldn't be the case if short format cricket has degraded everyone.
Jarrod Kimber also has a lot of evidence to show and support his points, which you've handwaved as batting being crap when he's refuted that already.
Yes? Did you just forget how good things have been set up in favour of both English pacers and good opposition pacers to run through sides with swing and seam? Statsguru shows New Zealand at the top of the average runs scored per wicket list, and England just above India in the lower half of the list:
Team records | Test matches | Cricinfo Statsguru | ESPNcricinfo.com
stats.espncricinfo.com
Yes, 7 bats in one generation averaging over 40 is an indication of batting favoured conditions rather than necessarily good batting on bowling favoured conditions. Things have changed quite a bit from consistent scores of 350+ in Tests, this is the reality.
The exact reason is pretty clearly laid out with proof though. No reason to not believe it.