There's a learning curve to NBA defense and to state that big men make their mark defensively on day 1 is patently false. Even the best coached defenders came from a different set of rules and get into foul trouble and have to learn to manage the game for a season or two. Why don't you cite examples rather than just claim to the contrary. You've been down on Bagley and made it your mission to go after him since you got here. I make it a point not to engage with you but when you throw out opinions that run counterintuitive to decades of basketball wisdom you should back it up rather than just state it as fact.
We have a list of 59 rookie bigs who played 50+ games, 1500+ minutes, and F/C, C, or C/F.
35 of them had a DBPM of +1 or greater in their respective rookie years. Almost all the defensive greats started making defensive impacts right away while it takes even most great point guards some experience before they can on offense.
—Duncan, Bol, Eaton, Hakeem, DRob, Wallace, Camby, Ewing, Mutumbo are all plus defenders right away. Big Ben makes the biggest leap as he started off modestly and then reached incredible defensive highs.
—Meanwhile, guards, especially small guards
will make their impact on offense and very few are ever impactful as rookies.
Additionally, very few PGs of typical height are ever even neutral defenders—they’re almost all -1, -2,-3 type defenders. Their offense hasn’t developed yet since they can’t run an offense yet so their offensive impact is either 0 or negative and most of them will be negative defenders even at their peaks.
Put all those things together and you get bigs making an impact much earlier in their careers.
—Lastly, it is nearly impossible to make
MORE of an impact with elite defense than with elite offense. Elite offense has a higher ceiling especially since you can make 3s instead of twos. The most impactful defensive players are Bigs and they will NOT be able to prevent as many threes as a great offensive player can create (LeBron, Curry, Harden).
Additionally, even bigs, who can affect opposing offenses the most, can’t take away every player who’s scoring on every possession while an offensive player can directly make an offensive impact on every play no matter who is defending him and isn’t limited like a big on defense is.
Elite defensive and offensive seasons comparison based on OBPM and DBPM
+12 seasons: 1 on offense, 0 on defense
+10 seasons: 1 on offense, 0 on defense
+9 seasons: 13 on offense, 0 on defense
+8 seasons: 15 on offense, 0 on defense
+7 seasons: 27 on offense, 1 on defense (right at 7.0)
+6 seasons: 49 on offense, 12 on defense
+5 seasons: 108 on offense, 38 on defense