Milicia line need be the counter unit of Ghulam?

You are talking like the game starts at late game. It is true that Hindustanis are strong in late game but before they can switch from camels/ghulam into HC, you have time to beat them (its super expansive to research infantry techs + chemistry + range techs + imperial castle tech for HC).

Do not forget that they have only camels and hussard in the stable (very weak vs almost everything, any infantry and even archers).

There major eco bonus is the cheaper villagers. So earlier you attack less this bonus is significant. To be honnest I do think Hindoustanis are OP. Ghulam has already been nerfed with le last DLC and the lost hallebardiers.

I actually think their biggest powerspike is in the midgame. Only the HC can only come into play in the lategame, but thatā€™s usually also the timing when you actually have to deal with a lot of infantry.
Ofc you can try to make infantry against hindsutanis in the midgameā€¦ good luck stopping their boom with that.

HIndustanis still have the 2nd best boom after Cumans btw. Even better than Poles. Thatā€™s also one thing we need to keep in mind. Their lategame itself isnā€™t necessarily the best, but they often just end up with a better economy there.