What all goes wrong as a v gets high mileage?

15k on a gen3 engine is hard to believe.

When the oil light comes on you have run out of oil, at least enough the oil pickup is running dry. Just as bad, oil provides cooling for the lower end of the engine in addition to suspending combustion blowby, carbon and dirt. Less oil means it is way hotter and contaminates are higher density.

If you get an engine that literally does not use any observable oil, keep it that way with 5k oil changes like your wallet depends on it. Same basic advice for coolant; every 50k or less, no exceptions.