Mechanically speaking, nothing is gained with a 15mm axle over a 20mm. What we gained by going with the (mechanically inferior) 15mm axle is the rapid and widespread adoption of thru-axles across all mountain bikes, then road bikes, then gravel and CX bikes. If the axle choices in 2012 (or whenever) were viewed as a binary choice between lightweight XC-oriented QR axles and a heavy DH-oriented 20mm thru axle, I think it would have taken a lot longer for product managers to go out on a limb and spec better forks and axles. 15mm axles gave product managers needed cover...