You are correct. But some things have to change/adapt with modernity.
In the past, we did not require language-based states because our kings neither had the ability, nor the will to go imposing their maatri-bhaasha as bhaasha of their rashtra or jaagir or whatever.
Languages competed in organic fashion and being more or less equal in all respects except demographic weight, stabilized according to their sizes.
but we live in modern world now. Every kid goes to school. learns in a given language. we dont send runners from one mantri to another with seal of authority and word of mouth - we send reams and reams of writing.
So WHAT language is chosen to represent the working language of schools/gormint matters now. because NOW, if tamils took over as lingua franca of kerala, malayali will go dead.
Before, 1000 years ago, if a chola rajan inherited a jagir from a chera rajan and came over in suddh tamil, it doesnt affect much more than 30 people in his court.
I am not 100% sold on the idea of linguistic identity being absolute for state-hood, because i can see it as a never ending routine of division and maybe even scessionism, but at the same time, i also realize that we must 'protect' languages in certain capacity as in lingua franca of gormint or schools, else they will go extinct.