Say that an English speaker speaks a sentence, and an English listener understands it. If they both agree that it was an English sentence -- including no jargon -- and they both agree on the meaning of the sentence, then that's correct English. If the two individuals have also never met before, then it's certainly correct English.
That's what "defined by usage" means. English does not have a language regulator or language academy.
English does have lexicographers, though. I should be able to turn to a dictionary to find out what "literally" means, and it's regrettable that that inquiry will tell me the word has two directly-opposed meanings, without noting that one of them is wrong.
I think you are mistaking the map for the territory. I think you are blaming the data (actual usage) when the model is wrong (dictionaries).
"Literally" is allowed to have a valid and true definition of "figuratively" because exaggeration and hyperbole are used for rhetoric and expression. That's a vital and popular way language is used, and in the case of "literally" it is so commonly encountered that it worth noting in descriptive texts that it's common.
FWIW that's also how languages work that do have an academy. The academy may be a participant in the process but they don't control it, that's basically just a relic of before we understood how languages work.
> that's basically just a relic of before we understood how languages work
Relics that unfortunately maintain significant influence in at least France and Germany.
When I grew up and was just learning to write, orthography was "revamped" by literal committee, in some cases even going so far as deriving a new spelling via false etymologies.
It wasn't really a big deal for me practically, but it just seems bizarre.
Say that an English speaker speaks a sentence, and an English listener understands it. If they both agree that it was an English sentence -- including no jargon -- and they both agree on the meaning of the sentence, then that's correct English. If the two individuals have also never met before, then it's certainly correct English.
That's what "defined by usage" means. English does not have a language regulator or language academy.