Indus valley didn't know the use of plough. They had a primitive way of cultivation by using fertile silt left by periodic flooding of the rivers. Artificial dams and barrages were used to flood the river. This very flooding, which was so essential for farming, might have been one of the causes for the decline of their civilization. Lack of better agriculture is a striking difference from Mesopotamia and Sumer. Indus Valley had no canals as in Mesopotamia. Much less surplus in grains might have been the reason for the lack of internal competition and also external attack. That might have been the reason too for minimal change over 1500 years. The Aryans destroyed the dams and brought greater areas under cultivation. RV mentions Indra destroying Vritra, which means restrainer, obstacle and may refer to the barrage that obstructs the water. Indra is said to have removed rodhas, meaning obstacle but used in RV in context of dams and embankments. The Aryans broke barriers between primitive and conservative tribes which had managed to preserve their archaic religion and tools for long. They introduced better production techniques. Several groups of people separated for ages were brought together. Various skills, which were local secret, became public, thus stimulating more trade and prosperity.
A similar thing would have happened in BMAC around 1900 BC when there was a sudden surge of wealth with the arrival of the Indo Aryans from the northern Arkaim and Sintashta Cultures.
The Aryans never displaced anyone or enslaved entire lots of natives. The Aryans brought in new techniques and knowledge and provided the much needed guidance that led to their prosperity by better harnessing natural resources, producing more and enhancing trade and commerce. This made the Aryans very much acceptable to the natives as their elites.
In most cultures the natives were always enslaved by the intruders. But in case of the Aryans there are multiple instances where the natives were promoted to the ranks of elites. Discussions on Peoples & Conflicts during Rig Vedic times have many such instances. In fact the addition of shudra as the fourth class was to accommodate the natives. It may be still argued that the natives were put in the lowest stratum of the society. But still that's much better than treating them as slaves.
Anyway, coming back to the topic of language takeover it's important to understand that such a thing was possible not through force. Rather it was done through amalgamation and cooperation. It's a very rare instance in the history of mankind that a few elites make such a positive impact on the lives of the natives that they eventually adopt not only the language but also the culture and religion of the former.