Of course it's guesswork.
Just because a batsman has failed once in bowler friendly conditions does not mean he will always fail in such conditions.
Of course they wont always & can improve. As aformentioned you can judge whether batsman X of this FTB era would have been able to average Y (whether higher or lower) in the 90s. Based on the few on 90s like scenario's (difficult batting conditions vs top quality pacers or spinners) that batsman X would have encountered in this just 2000s era.
The example i always give with Matt Hayden. He was a major FTB between IND 01 to NZ 05 (although he had lil drop in form from IND 04 to NZ 05 in between), hardly ever facing a quality pace attack or encountered a pace bowler friendly conditions in those 4 years (except for Ashes 01 where he struggled).
Then when we came up againts a quality pace attack in Ashes 05, he was exposed technically & his career was pretty much dead after the TB 4th test. If Hayden didn't reinvent himself out of the "bully mode" he wouldn't have scored that Oval hundred his test career was done at that point. This improvement also enabled him to scored runs vs the very good SA attack home/away in 05/06.
So based on this that improvement its fairly safe to assume the "reinvented Hayden" would have done well in 90s. Just that i dont believe he would have averaged 50.
Then we have Langer. A joker againts spin in the early 2000s always was far moe efficient againts pace given he was from Western Australia. When AUS toured IND 2001 he was throughly exposed. Later after he reinvented himself into a complete batsman & opener he managed to one of best innings i've ever seen againts spin in the sub-continent againts in SRI
2004.
While on the opposite end of the spectrum we have many other batsmen who have pounded poor/good attacks on roads & when confronted againts very good pace attacks in testing conditions they either have failed miserbaly/very below par againts quality attacks (Sehwag, Jaffer, Gayle, McKenzie, Sarwan, Cook, Bell, Yuvraj).
Or have not been as dynaminc againts the very good attacks as they where againts the joke/poor/average attacks on roads (Mohammad Yousuf, Samaraweera,, Smith, Younis Khan, Gilchrist, Collingwood)
^^ These cats AFAIC would have struggled or wouldn't have averaged as high as they did in this 2000s era if they played in a more bowler friendly era. Based on how they fared overall againts both facets of bowling.
The elite group of batsmen IMO who where equally good on roads as they where in bowler friendly conditions during the 2000s era where just Ponting, Dravid, KP, Kallis, Sangakkara, Lara, Tendulkar (after his tennis-elbow woes eased), Chanderpaul, Laxman, Langer, Martyn, Clarke.
^^ But of this list IMO only Ponting, Dravid, Kallis,, Lara, Tendy, Sanga as the title of thread goes deserve to average 50.