The problem is that the Greek language was only spoken by a tiny, tiny, fragile elite, and it never gained much status at all. The Indo-Greeks learned the local languages to speak to their people, not the other way around.
For the record, I don't disagree with any of this. But for the purposes of discussion...
Maybe Greek could settle in as both a trade language (via the greater Hellenistic world) and the language of the ruling elite. Then have Bactria or the Indo-Greeks adopt a policy of promoting and encouraging lower and middle ranking personages lacking the social foundation of local notables. In tandem with that, have them seek to educate the children of the elite in the quasi-Greek language of the court, with said children serving as both hostages to their parents good behavior and vehicles of Hellenization.
Learning and speaking Greek thus becomes a token of power and a symbol of one's rank or education in a vaguely similar way to how the Roman elites absorbed the language. Over time, Indo-Greek mutates and absorbs words and accents from the local languages, becoming a distinctive dialect of its own.
Unfortunately, it's hard to see this lasting much longer than the political structure that builds it, especially in the Indus valley. Once the kingdom(s) collapse, the far more numerous non-Greek Indians will simply wash away the thin Hellenistic veneer. Even so, Greek would have lasted a bit longer, and contributed a bit more to regional languages in such a timeline.