It should be noted that though "myriad" comes from a Greek word for 10,000, and this is why the term "myriadic" is used to describe a number system based on powers of 10,000, the word itself is common in English with no meaning other than "a lot", and can't be interpreted to mean 10,000.
‘I thought it looked a little queer. As I was saying, that seems to be done right—though I haven’t time to look it over thoroughly just now—and that shows that there are three hundred and sixty-four days when you might get un-birthday presents—’
‘Certainly,’ said Alice.
‘And only one for birthday presents, you know. There’s glory for you!’
‘I don’t know what you mean by “glory,”’ Alice said.
Humpty Dumpty smiled contemptuously. ‘Of course you don’t—till I tell you. I meant “there’s a nice knock-down argument for you!”’
‘But “glory” doesn’t mean “a nice knock-down argument,”’ Alice objected.
‘When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean—neither more nor less.’
Of course, this is a famous example of an idiot who can't communicate. (Or, indeed, subtract 1 from 365.) Words mean what they mean by common agreement; your wish to use one inappropriately will not make that use appropriate.
> If I said there are five myriads of sand grains in an hourglass, what could this possibly mean but 50 thousand?
That you're given to poetic turns of phrase? That you're confused about how to refer to numbers in English? If I actually encountered this phrase, I'd lean heavily toward option one there. It would be much like saying the glass contained five times as many grains as the stars in the sky, instead of as many.
Nothing to apologize for. Try finding an example of sense 1. The dictionary you want to cite doesn't provide one.
By inspection, there are exactly zero such examples in the first 200 COCA hits for "myriad", of which there are about 4000 total.
There are 72 total hits for "myriads", of which exactly one is arguably using the sense of 10,000, but it is a translation of the foreign phrase 八百万の神, where 万 is literally 10,000. I don't know whether the number in the Japanese phrase is meant to be interpreted literally, but I doubt it. Even here, the translation given is "myriads of gods", not "eight hundred myriads of gods".
You don't seem to understand what linguistic prescriptivism means. Everyone agrees that there are rules determining what is and isn't valid in a language. Descriptivism is the approach of determining the rules from usage. Prescriptivism is the approach of postulating an authoritative source.
Descriptively, "myriad" cannot be used to indicate the quantity 10,000 in English, only to indicate a large but vague number.
