But the future is longer than a few decades, so why extend the mistake further?

Because change costs way more to than you think: maybe billions of dollars and impacting billions of users.

Creating 2 different systems carries risks: it has a learning curve, a support cost, and breaks everything that came before.

Perhaps, in a utopian green field, we redo a systems language, a portable systems API, and a common operating system with a compatibility layer to the old way on top.

Change has already partially happened since there is no one single filesystem with one behavior.

Not changing impacts many more users (all the future billions) and costs even more: the learning/support/etc. is all there, and it's harder to learn about a buggy system since you have more papercuts

Also, it doesn't break everything, we're not living in a dystopia

This is all true. We should also fix global warming, it's the same kind of problem. Massive investments now or even more massive losses tomorrow.

1) Not everybody, or even a majority, agrees that the current state is a mistake

2) Change what to what? There are multiple filesystems systems (and OSs) with different and sometimes incompatible filename encodings. Who picks the winner?

3) There is irony on dropping backward compat and requiring UTF-8, an econding whose claim to fame is backward compatibility with ASCII based systems (and not coincidentally was designed by UNIX Elder Ones)

I was responding to your specific argument ignoring the bigger future for the smaller past, not sure how the broader questions about what others think etc is relevant

1) so? That will be true for anything

2) sequence of bytes to human encoding. Who picked the loser??? This would similarly be some unidentifiable group of people. You can start by picking the winner for yourself, even at a conceptual level

3) no irony, sequence of bytes is not ASCII