.Net isn't platform specific anymore with the introduction of .Net Core.
And it was "not" with mono before it. I am not seeing the thing that will change that though.
c# is not a bad language as far as languages go and you can certainly make programs of nearly all flavours in it, I doubt we will ever see it outshine web development stuff or go for embedded stuff or go for phones worth having if you want more than a baseline phone.
I am not sure that for most purposes I could even suggest C# over python.
That means,Is Programming language useless in future
Until I have a star trek style "computer solve this abstract problem for me" style programming/AI/interface then programming as a distinct concept from maths, logic and what have you will not be useless. I would be pretty stunned to see anything close to that in the next 30 years. Thus if you are contemplating it as a career or useful skill to have in life then yeah it will be. It will change but I don't think we will see universal computer literacy during that time either -- they had their chance, they taught everybody microsoft word instead.
What will happen is it will become stratified even more than it is now. Take a person that has just gone through Java school and they will happily be able to make something that converts formats, speaks to a database to return a nice graph for someone, make a program that calculates something from a bunch of input data, formats something in a certain way... which is what the vast majority of computing is concerned with nowadays and what most people in the real world want. All the really fun stuff I know about algorithms, low level electronics, video encoding, cryptogaphy*, assembly programming, systems architecture, low level operating system concepts and so forth matters far less for most purposes. There is still room for it but once where every actually computery type would have been able to hang there then now it is not the case.
Or if you prefer being able to communicate in a language is pretty vital, being able to succinctly describe what an adverb is would be of less importance to most things.
I doubt computer programmers will be able to carve out a niche like lawyers and accountants have today where you are almost legally mandated, if not outright legally mandated, to have one in certain countries for a lot of purposes. My impression of US business is if you have an idea on the toilet then if you don't have a lawyer involved before you have wiped your arse then screwed up royally. Back on programmers then that is probably more due to a massive opposition to any kind of mandated professional competence thing like the lawyers have the bar exam.
*not so skilled programmers trying their hand at cryptography is the cause of a lot of failures there, however few seem sufficiently motivated to bother trying to restrict/govern its use like they do for certain machines, chemicals, biological agents and similar such things.