See, that isn't even necessarily true, though. Though Turkic dynasties like the Ghaznavids, [post-Afrighid] Khwarezmians, and Seljuks did put an end to the Iranian intermezzo, that really only affected the ruling class and not the general quality of life of people in those lands. Often they actually patronized Persian cultural works and were most of the time content to let their subjects be [in Afghanistan not India in Ghaznavid's case]. For example, the Shahnameh, the most Iranian piece of literature ever, was completed under Mahmud of Ghazni's patronage, even if he was a bit of a dick about it.
It was Genghis and especially Timur that did a huge number on the region. Those two specificially, successors such as the Ilkhanate in Genghis' case and Ulugh Beg in Timur's case actually tried to put things back together.
Also, in sort-of defense of the former, though his destruction of Merv was famed, it actually was decently rebuilt later on until Timur permanently destroyed it. Most of his destruction is exaggerated (supposedly killed 1 million people in Nishapur -> persia had an estimated population of 10 million -> 1/10 of persia lived in 1 city?) and since he was pragmatic he didn't quite destroy everything, but his absolute decimation of Khwarezm is still unforgiveable.
t. Timur salt anon