Not everything on Linux is case sensitive but that's sort of a moot point anyways because its not an operating system question when it comes to programming languages anyways. File systems can have case sensitivity, I can certainly attest to that. Even you are admitting that c on cpm had the possibility of case sensitivity (it really needed to if it was going to have a shot at compiling existing source). I don't really putz around as much with postgres as I like but I don't remember it being case sensitive on Linux. Likewise with Fortran on Linux with the obvious exception of include files (just from what I've read, I haven't coded in fortran on Linux.) I think the key here is that you really don't want to change case sensitivity in programming languages simply because you want to have a chance at using / porting existing source code and in any case, you want to appeal to the language users and that would be a pretty drastic change to sell. Same for java and other languages which often use the case distinction to use between classes and instances of those classes (a programming motif I seem to remember seeing).
MS-DOS had at least two c compiler offerings that I had experience with, so I don't think there was any "trying to absorb the C variants" when it came to windows, from what I've read 1.0 had a c sdk already, although I never used it, I only had any experience using c on windows starting with 3.1, but I'd find it hard to believe if 3.1 didn't have parts in c. From what I've read, even 1.0's file manager was written in c. Simply put, c is case sensitive, its part of the language and by the time windows came around, c was enough of a thing that there wasn't going to be some sort of great breakage of any sort, simply put, operating systems just don't lock you in to case arguments with of course the caveat of the file systems, and even then, c isn't going to change your file system's case requirements to the best of my knowledge.
With Pascal being case insensitive (if I remember correctly), it'd be a bad move to make it case sensitive on Linux right? What about python? Its getting popular on windows with data folks, did it lose its case sensitivity along the way? (Honestly don't know LOL but I am tending toward thinking its still case sensitive to some extent).
The new aspirants for the systems programming language choice include go and rust, go is case sensitive, rust is "case complainy" lol but still these are independent of the OS. I'm betting google doesn't put much search computing power on windows servers but they don't seem to have problems with dealing with case insensitive searches right?
I just think its more that case sensitivity is going to be part of any particular languages experience and culture, and to think either is inherently evil is just sort of unproductive in my opinion. If one or the other takes a bit more effort, just toss it in to the programming languages merits and drawbacks, weigh them all and see if the choice is a good one. Honestly zero's origin falls into the same case in my opinion, plenty of my experience is with zero's origin but that doesn't mean I can't handle 1's origin.
As far as the user is concerned, they're still going to capitalize proper names, beginnings of sentences, etc so in their case case sensitive it is! I'm sure an email package that throws your carefully composed and professional message into all caps might encounter some confusion heh