Although the Himalaya Mountains are still growing today, eventually (regardless of whether the collision between the Australian-Indian plate and the Eurasian plate continues or not) the mountain will stop growing. Why is that

This is because of isostatic adjustment.

Explanation:

The mountains of the Himalayas keep growing because of the isostatic adjustment. it allows for a gravitational balance in the Himalayan mountain region, which causes tectonic plates to float below these mountains. This fluctuation occurs in the densest part of the asthenosphere. This density, together with the force of gravity, causes an increase in the height of the tectonic plate that sinks more and more. As the tectonic plate sinks, inversely, this plate rises, when its weight decreases, and it is this rise that causes the mountain to grow.