If the foreign intervention aren't willing to put their bodies on the line at the end of day wouldn't a ruthless counter-insurgency campaign succeed by running through the available pool of rebels?
Hypothetically, sure. However, the rebels in Afghanistan had a few things in their favor that leveled the playing field.
1) Sanctuaries inside Pakistan that the USSR could not touch.
2) Foreign fighters, specially Arab muslims.
3) Weaponry that provided the great equalizer in many situations, such as stinger missiles.
4) Lots of money and logistics outside Afghanistan, thanks to the CIA,ISI, etc...
5) Equal ruthlessness. The Mujahideen killed children for joining the Young Pioneers, for crying out loud.
6) Overwhelming local support, which made for a great recruiting tool.
Despite all this, OTL USSR arguably managed to leave a competent Afghan army to fight in their place for a few years. They were never able to quell the insurgency though.