He's an average centerfielder, which is way more valuable than an average first baseman or even an above average one. There's a reason that usually the least athletic players are put at first base or left field, and not center. It's why Gardner usually has had such crazy defensive numbers, he's a centerfielder playing left so he gets to way more balls than the average leftfielder. Pujols is still able to play first even though he can barely move these days.
Also, do you not believe that seasons have different conditions that affect offense? Like you believe in 1993 and 2019 that every player just decided they should be better batters? Nothing happened between 1968 and 69 that made offense jump? Players in Coors hit exactly the same way they do everywhere else?