They're not necessarily useless, they have their [limited] place. The crux of it is this: the ones who have high-level scoring ability + the appropriate shot/pass balance that maximizes their offense's productivity and efficiency, are the most conducive type to winning. This goes beyond the #s, and you don't even need some amodal basketball perception - just basic common sense.
We shouldn't be looking at this in a vacuum. The fact of that matter is, how many points and how efficiently an offense scores those points are mostly influenced by the player whom controls the ball the most, in this conversation that would be the PG. It's not just about how many points the PG scores, or how efficient the PG is (because that it all depends on the context of the game, the personnel and scheme), it's about how the opposing defense is treating the PG and therefore how they're treating the offense as a unit, and the opportunities this gives the offense to score. Just look at some of the things it affects:
the spacing (how a PG pulls in defenders all across the court and gives their teammates more room to operate with)
the help defense (how a PG pulls in defenders, rips defensive schemes and gives teammates easier scoring opportunities and/or mismatchups)
defenders rotating (how a PG forces teams to rotate)
the rhythm, confidence and belief of defenses (how a PG can get a strangehold on the control of possession flow, limit the defense's confidence by making it harder and less predictable for them to defend)
defensive matchups (how a PG can get a defense to mentally and physically overcompensate by being an equal shot/pass threat - teams using better guard/wing defenders and how it affects the awareness of other defenders of where the PG is and what they're going to do, and how it affects their mental ability of being concerned about another player while their own defensive assignment)
the mental and physical strain (how a PG can
break a defense mentally and physically and the domino effect it has on the opposing team's offense, how much energy and willpower they have throughout the game, how the opposing team's gameplan changes and lineup changes etc etc)
The list is much deeper than that. You then apply all those things to how they affect the PG's personnel on offense/defense, and their offense/defense as a whole.
Again, just to touch on this briefly - typically a center isn't the main ballhandler whereas the PG is, so therefore a PG influences the offense more than a center does. Just as a center is typically the main defensive anchor whereas the PG isn't, so therefore a center influences the defense more than the PG does. Take a look at Magic for instance, he wasn't a great defender by any measure (often was hid on defense and put on lesser offensive threats), yet he arguably had more impact on the game than any other PG in history and is the consensus GOAT PG, simply for his offensive impact (maximizing his offense's productivity and efficiency by balancing when to shoot/pass).
Nah, this is constant throughout NBA history. It's only more realized now because of the data/information available and the improvement of defenses/offenses, and how teams are more concerned about results and forsaken the archaic plot of traditional basketball.
See above.