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.