I’m not sure, alas. I’m only a hobbyist. It seems that improved brushless motors is a direct follow-on of the mass production of high-strength rare earth magnets, the motors being my main point of contact. (you can also draw a straight line from this to the rapid development of drones and quadcopters when combined with high capacity batteries, as you pointed out. )

In my experience, pinning down exact comparisons between brushed and brushless motors is a source of frustration since brushless torque curves behave counterintuitively at low speeds, e.g. the second chart you see here:

https://www.fingertechrobotics.com/brushless-torque.php

That behavior is why you can’t find a torque vs current draw curve for brushless motors. You can get an estimate based on kV values, but again, it is very speed dependent.

In terms of practical experience, and for speed ranges that you’ll see IRL, brushless tends to give you substantially more torque per unit mass while running quietly and with higher efficiency to boot.

I see it on my current bookshelf project, and I’m cheap so my tools are low-end stuff I could scrounge or find on sale. My old-school brushed Black and Decker 18v drill chugs when drilling holes in the old recycled hardwood we’re using for the project, wheras the brushless Ryobi 18v cuts through like it’s nothing.

Based on what I’ve seen in my fighting bots, a drop-in replacement would get you get roughly a 50% boost in torque for a similar weight of motor at the speeds concerned, unless you wanted a low speed brawler and didn’t want to gear the motor up much, in which case stick with brushed.

Sorry to carry on, it’s a whole thing!