I dunno, if not for dudes like him maybe we'd still be speaking old English.
If Tolkien thought it's better for it to be 'Elves' rather than 'Elfs' then i am inclined to trust him.
Old English hasn't been spoken since before the Viking era; England was speaking
Modern English by the time it set up its American colonies. Beowulf is written in Old English, Canterbury Tales is Middle English, and Shakespeare is Modern English. Technical terminology aside, you're wrong in that Tolkien wasn't really pushing language
forward; he kinda went out of his way to make his writing style something he considered timeless - no "thou hast" but no "hey guys" either - and it's debatable how much influence he actually had on the way people speak today, but I'd say English by the 21st century had gone in some ways the opposite of how Tolkien preferred to write.
Now, throughout the 1800s and some of the 1900s academic culture was overtly trying to make English more Latin - I myself had a teacher around 2007 or so tell a whole class that English was part of the Romance language family, when pretty much every linguist today will tell you it's Germanic - and you could say that Tolkien was a significant influence in pushing back for a more Germanic English. If you read other famous ME authors, especially Poe, you'll notice that their
extensive educational origins incentivized the accumulation and utilization of exceedingly multi-syllabic and almost exclusively cis-alpine vocabulary, whereas Tolkien mostly liked to use good, simple, everyday Germanic English words, which makes his stuff easier to read for common folk even today, but could make him sound a bit old-timey, especially when he throws in already archaic words like "whither" and "thither" and "hitherto." It's why Tolkien's world has a Race of Men instead of a Species of Humans - same meaning, different linguistic origins - but most post-Tolkien fantasy worlds have a Race of Humans, taking some of his style for certain things and going the opposite direction for other things.