That's interesting, however, I can think of three examples at least where this is not the case by design.

In every instance of the above, one pulley is arranged to have moveable flanges for tensioning and the (correct) belt would never approach the flange edge until utterly worn out.

The three that spring to mind are:

WWII Morris Commercials and all the civillian cars/trucks using the same engine type; in this instance the front water pump flange is threaded on the shaft and can be screwed back to increase the operating radius and therefore belt tension.

All pre-war, war-time and post war flat four VW engines have a shimmed dynamo pulley, taking one shim from between the separate pulley flanges and "storing" it as a packing piece on the outside of the flange also increases the pulley operating radius and therefore tension.

Mk.2 Alvis Stalwarts have twin fans with two belts arranged in a Y formation, the junction of the arms would be the waterpump and the vertical line represents the toothed belt drive to the pump from the crankshaft. The fan pulleys are on stepped diameter shafts having left and right hand threads; loosening the axial clamp bolts and rotating the fan will have the flanges move equally inwards or out to set the belt tension. With the fans on a separate housing unit, turning the flanges in opposite directions allows them to be aligned with the water pump driving pulley vees.

R.