That's actually not so weird. The 8042 simply had an unused GPIO. The other option would have been to put a whole peripheral on the bus for this purpose, to the tune of like 3 chips and $40 of retail price.
The point was just to have a compatibility hack for old apps, not to pollute the architecture going forward. The reason that pollution happened was actually that the PC clone makers incompletely and incorrectly copied this feature, so later OSes never knew a priori whether or not it would be enabled by the BIOS, and they all had to have the same 10-20 instruction sequence to turn it off. And that meant that future PCs needed the same 8042-looking device on the bus to handle that request.
The point was just to have a compatibility hack for old apps, not to pollute the architecture going forward. The reason that pollution happened was actually that the PC clone makers incompletely and incorrectly copied this feature, so later OSes never knew a priori whether or not it would be enabled by the BIOS, and they all had to have the same 10-20 instruction sequence to turn it off. And that meant that future PCs needed the same 8042-looking device on the bus to handle that request.