45 doesn't provide anywhere near the amount of energy of penetration as a 10mm
True, but adding a recoil buffer, a steel guide rod and a 24lb spring to a G21 will allow you to run 45 Super - which does match 10mm in velocity and ft/lbs while still using the stock Glock G21 barrel.
Those parts run $35-$45.
Running 45 Super avoids buying a $100+ 10mm barrel for the G21 or buying a 10mm specific G20, and 45 Super ammo costs noticably less as a bonus.
45 Super is 98% of 10mm performance at 1/3 the cost.