You'd be surprised...
M has been the accepted prefix for 1,000,000 since the 19th century, widely used in science, networking standards and disks, decades before Apple made any change itself...
As was the common convention, all advertising materials, all reviews, all documentation and all common understanding was that the 64 in its title represented 64KB as in Kilobytes.
This same convention, for MB = Megabytes, and GB = Gigabytes and all of these representing powers of 2 (no matter how "incorrect" that may be), was proliferated through the overwhelming majority of advertising, commentary and documentation both previously through the 70's, and through the 80's, 90's and 2000's.
This was the convention commonly understood for the many-decades-long duration of the birth of modern computing.
To attempt to change it after decades of intrenchment for the "technically correct" version and gain loads of confusion, and almost nothing in return, is a bad value proposition.
As countless others already wrote here, that (and all your examples) where the case (and remain the case) for memory.
Disks, network, etc, have used the powers of ten since forever...
Hard disks are not floppy disks or memory, they always used base10 since half a millenium, and Apple just continues the same, like everybody else.
End of non story.
To borrow your C64 example, the most popular harddrives to add to a C64 were Lt.Kernal(Fiscal) drives. The first model, sold as 10MB, was 256 bytes per sector, 32 sectors per track, for 1224 tracks. Giving us a raw capacity of 10,027,008 bytes - 10MB or 9.5MiB.
You're talking about it being a convention in home computers when the vast, vast majority of them simply didn't have harddrives - While the machines that did have harddrives were already using base10 (an RK05 was 1.2Mwords - 1,200,000 words. Twice the RK04's 600kwords).
The convention has always been base2 for RAM and base10 for storage. And yes, it's always caused confusion. Just as the convention has always been bits/sec instead of bytes/sec for linespeed, even though this also causes confusion to this day.
The awkward truth is that none of this was ever intended to be consistent. From the day we started selling computers to people who weren't 100% sure what these numbers meant, it's always been "close enough".
If we may now agree that the original, widespread understanding of a KB was of 1024 bytes, and a MB of 1024 KB, and that only later did a widespread impression start to be spread that actually a KB|MB could be 1000 instead, we strike at the very heart of this issue:
Adopting an alternative "standard" spearheaded by particular (and not all) storage manufacturers incentivised in the first instance to use the previously well-established terms KB and MB duplicitously in their favour, strikes me as the worst way to resolve any consumer confusion surrounding it.
Except when it, of course, isn't. $10M is certainly not ten megadollars.
Even science isn't that consistent about it. The SI base unit for mass is the kilogram. Yet we don't have megakilograms. We have megagrams, but nobody uses them. Instead we use tons. Somehow kilotons and megatons are used though.
If we're willing to accept the above then why couldn't we have stuck with the powers of 2? Digital information at some level is going to become base 2.
Money is not covered by S.I./metric units anyway, so not sure what's the point of this argument.
And even at that, while M here is the "mega" unit, the value 10M is still 10 million (1010^6) dollars (not 102^20 dollars).
M$10 looks kind of clumsy, though.
The unit of currency is the dollar. There is no units of currency that corresponds to 1M dollars.
The SI system of units devised a systematic way to define derived units that reflect a certain amount expressed in the base unit.
Currency don't have the benefit.
This reminds me of something I was unaware of until very recently; the decibel (dB), is one tenth of a Bel. The discussion was that you'll never confuse a B(byte) with a B(bel) because we never specify 10dB as 1B, and we almost never need to specify a 10th of a byte.
But back to your point - we use base10 for everything but RAM. So if we'd stuck to one system to avoid confusion, RAM probably wouldn't have won.