This key is 'fit'.
One could, and probably should, write a long learned essay on the wrongs created over the last 150 years under the delusion this sentence makes grammatical and real world sense.
When, however, we modify the sentence so it reads: "The key will fit this lock but not that lock" , most of us agree it now does make grammatical and real world sense.
The entire phrase "will fit this lock but not that lock" can be thought of a one long adjective modifying ,and accurately limiting, the noun key.
The outstanding aspect of that long adjective phrase is its tentative nature -- which, in turn, accounts for its windy lengthiness.
"Will" could be replaced by words like "used to" ,"once", "may", "no longer" and the words "this lock" and "that lock" replaced by other modifying and limiting nouns.
But the phrase "the key is fit", with the word "fit" being totally unmodified and unlimited by adjectives, together with the fact it is set in a tense of eternal and universal is-ness , strikes us as very odd indeed.
Unless we modify the sentence to say "John is fit", then most of us accept this sentence as seemingly making perfect grammatical and real world sense (and tense).
But it does not.
That broad shouldered six foot tall 175 pounds hunk of svelte eye candy might be "fit" in all of our eyes, but is he actually "fit" for being a race jockey or "fit" to crawl into a narrow tube to weld a joint ?
The Darwin of 1859 said that in a real world of 'the survival of the fittest', strong but lithe men would become horse jockeys but not Rugby forwards while huge chunky men would become Rugby forwards but not race jockeys.
In our actual world, the reality we must live with, "Fittest" is always found modified by an adjective phrase , indicating the particular time and space limitations that allows this particular being or object to be temporarily the fittest for that situation.
It accepts that the world is filled with millions of possible niches and that they change all the time.
I don't think there is any possible moral or scientific objection to this Darwin.
But the later Darwin of 1871 seemed to imply that reality is really about the survival of the "fit", an unmodified, absolute and universal/eternal noun : European males being "the fit" and no one else - and nothing else - being in that category.
It sees the world (and eventually the universe) as potentially one great vast niche, with European-origined humans as the only species needed to be able to fill it completely and permanently.
Modernity science fiction saw future human worlds as living under glass bubbles on planets of bare rock, devoid of atmosphere, generating all we need by chemical synthesis, with no need for plant or animal or microbe.
No need for Jew, Gypsy , Slav or 'defective' either.
Those authors and illustrators only said in print and pictures what our grandparents (and the Darwin of 1871) were just thinking.
Until 1939-1945, when they got a chance to play it all out in a world-war sized sandbox ---- and ended up with sand in their Pampers .....