Not every cartridge exists to burn barrels faster and recoil harder than the previous one.
Ding, ding, ding, we have a winner - new sig line material, lol. But to try answer your questions more specifically:
It's niche is EVERYTHING. It's the perfect goldilocks cartridge, arguably. It can do it all with one cartridge, without excessive recoil, blast, and ammo expense. I'll guarantee you that a 160/162 grain premium bullet from a .260 rem will penetrate the brain and kill the largest bull African elephant you can find, in the hands of a very skilled elephant hunter. With light 80-95 gr bullets, it's an excellent varminter as well, and everything in between. Anything that can kill the largest living land mammal (in a pinch) and also serve as a varminter is an excellent all-purpose cartridge choice. It's obviously also great for self-defense versus human predators.
Or you might say it's niche is *deer hunting*. It's the perfect round to cleanly kill deer/antelope/sheep/goat sized critters without unduly beating yourself up and/or excessive bloodshot meat, and can be packed into a LIGHT rifle which can be humped a long ways.
The 264 Win Mag was a great cartridge and certainly would outshoot the 260 Rem, so what would make the newcomer any better?
It's a lot better because it's quite a bit slower, so your barrel will last several thousands of rounds instead of several hundreds of rounds, before accuracy degrades, and does almost the same job with a lot less recoil, blast/flash, and ammo expense.
Anyone try any penetration tests of the 260 against a 7mm or 30 cal with bullets of similar sectional densities and velocities?
No, but the heavier bullets' (in the 139-162 gr range) have sectional densities equal to the best 7mm bullets, and together with 7mm, are second to none.
Do its long bullets handle the wind better than other calibers?
Well mostly yes, and a little bit no. You can find slightly better BCs in 7mm bullets (and BC is what bucks the wind, with reduced TOF), but it's the *combination* of BC and muzzle velocity which is king with certain 6.5mm bullets. You can push an almost-as-good-BC bullet significantly faster than the highest BC 7mm bullet can be pushed with with similar cases, and therefore have better *overall* performance in terms of drop and wind-bucking.
Moderately-low to moderate, as hunting rifles go.
Would you rate it a good choice for a youngster (say 12 years old) just beginning to hunt?
Yes, only a few better.
