Xue Sheng
All weight is underside
For starters, there are many weapons besides the fist.
Secondly, if you choose to use your fists, it is still a bad idea to aim for hard targets, because the hand is a fragile sack of tiny weak bones.
Hitting trees or walls may condition your skin, nerves, and even thicken up the knuckles from microfractures, but it also causes cumulative damage, with an ever present risk of permanent damage.
Good luck playing the piano when you are 50. I recently saw a documentary about some old-school karate Masters in Okinawa, clubs for hands but not good for much else. Hands mostly fused together from all the fractures.
Hitting bare knuckle with maximum effectiveness without injury is more a matter of form and technique than surface or impact conditioning, which comes at a greater cost/benifit ratio by hitting softer targets like heavy bags (with or without hand protection) than from bashing your hand on unyielding surfaces.
Had no problem playing anything when I was 50.....I'm almost 60 and my hands are fine, don't play piano, but ukulele, mandolin and even guitar I can play without issue, and I have been training this way since I was in Junior high school. It is not just pounding away with your fists, it is knowing how to hit with your fists.
Strangely enough, I have arthritis in my knees and hips, but my hands are just fine...