It's irrelevant whether a film was mixed in mono for budgetary reasons, or due to technical limitations, or because the director only had one ear.
What matters is that the process resulted in a distinctive and unique soundtrack, representing hundreds or thousands of individual artistic and practical judgements that can never be reproduced.
Such a soundtrack has undoubtedly deteriorated; extraneous noise has been added; high-end frequency information has been lost or attenuated. This is what the process of restoration is for. Not to change or update but to return the soundtrack to what the director and sound editor heard when it was all new.
The same can be said for any original soundtrack, no matter how many channels it had. This thread has been skewed by its focus on monaural sound; the important issue is the preservation of the original sound, whatever its format.
Newly constructed multichannel soundtracks are fine (well, except when they intrinsically suck), and if they help widen the audience for older films, that's great, too.
But this is just another front in the battle between the studios, who are dedicated to seeing everything as "product," and us, the audience, whose only power is the power to choose for or against being treated like we're stupid and ignorant.
Even if we end up preferring the remixed sound, the comic-book "pop" video, the reinvented subtexts (Han Solo is an unmitigated good guy who would never shoot first), we should still be royally PO'd when we are not even allowed to see or hear what's being changed, when the original versions are withdrawn from distribution and only the plasticized and sanitized replacements remain available.
This is an infinite door, that can keep opening wider and wider if we simply accept it. Every film, TV show, or song that you ever loved will become suspect when reissued, because you've told the studios that you don't care about the changes they make.
On a higher level, it's worth mentioning that in the modern world, much of our culture is bound up in movies, TV, music, and other electronically stored and reproduced media. I am very uncomfortable with the notion that significant pieces of our culture can be manipulated and significantly changed by private interests whose only motivation is profit.
But that's part of our culture too, and the only way to fight it is in the marketplace. If adulterated titles fail to sell, and enough peeople make it clear that it's the adulteration that's hurting sales, things could change.