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Why do continent-continent collision mountains eventually lose their mountainous relief?

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A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

Mountains formed by continent-continent collision eventually lose their mountainous relief primarily because they possess a finite volume of rock that is not augmented after the collision occurs. Unlike mountain systems associated with subduction zones, where rock mass can be added over time, these collision-type mountains rely on an initial, fixed amount of material.1

Over time, this relief is reduced through several processes:

Erosion: This is the principal agent of destruction. Because the initial rock volume is limited, these mountains are subject to long-term denudation, which gradually lowers their elevation.1
Tectonic Processes: Beyond erosion, horizontal crustal extension and associated crustal thinning can reduce and eliminate the crustal roots that support the mountains, causing the belts to widen and their mean elevation to diminish. Additionally, the cooling and subsequent thermal contraction of the Earth's outer layers contribute to a reduction in average height.2

Geologic evidence supports this, as no mountain structures of the continent-continent collision type older than 500 million years retain significant mountainous relief.1