Sure. 'Lovers' isn't contemporary English at all, is it?

It is, but it's more restricted in usage than it used to be. One might say "the notorious criminal Alice was captured last night, thanks to testimony from her former lover, Bob", or something along the lines of "Catherine the Great's numerous lovers"; but one wouldn't say "this is my lover, Charlie", and "Dennis and Eve are lovers" would sound stilted, if not exactly incorrect, in most situations. I get the impression that it's now used with indirection and a slight pejorative air, where originally it might have been a direct, neutral description of a relationship.

("This is Charlie, my loooover" is a possibility in some dialects, but that construction emphasizes the relationship by drawing attention to the archaism.)

Hell, it wasn't even considered committed in the 80s. Although I suppose different regions may have changed faster, in the South in the late 80s/early 90s, "going out" was what we said for "going steady", while "dating" implied a more casual relationship. (And the actual term 'dating' was rarely used - I remember being asked, "you guys messin'?" after a couple dates with a boy.)

So yes, true.

My memory of sitcoms and comics from the '50s agrees with you.

That's all that I have to go on; I wasn't alive myself back then.

All the same, I still think that ‘lover’ is a contemporary word. A bit old-fashioned, and usually singular, but I was alive for a time when a gay man could introduce another man as his ‘lover’ and it would be perfectly natural, with no other word that would mean quite what he wanted to say. (Now he could say ‘fiancé’ or even ‘husband’ and that would seem natural, but once upon a time it wouldn't have.)

ETA: Also Nornagest's ‘former lover’.

koibito 恋人 vs. aijin 愛人 -- so it's only half identical. 人 has kun'yomi hito (from Old Japanese *pi₁to₂), with voicing of the initial consonant in the compound word koibito, and kan'on reading jin. If Wiktionary can be trusted, koibito is the generic term for 'lover/boyfriend/girlfriend', whereas aijin was borrowed (regularly) from Chinese to translate the English terms 'lover' and 'sweetheart', underwent semantic shift in Japanese, and ended up meaning 'mistress'.

Interestingly, Chinese 愛人 àirén is just an old-fashioned word for 'lover', and the word for 'partner in an extramarital relationship' is 情人 qíngrén... except Valentine's Day is qíngrénjié. Wiktionary also thinks there's a difference in usage of 愛人 àirén between the PRC and the ROC, but it doesn't describe it.

(Why was rén borrowed as jin? I'm guessing there are borrowing patterns, like how English has borrowed enough from Latin that new Latin borrowings will mangle the vowels in entirely predictable ways, but I don't know what they are. My first guess was that kan'on readings are based on a dialect of Chinese that had the same ȵ > ȵʑ > ɻ shift as Mandarin. I figured that was too simplistic, but given that 日 has the kan'on reading jitsu and the go'on reading nichi (go'on was earlier than kan'on), it might be right. Aijin is almost certainly regular, since 刃 is rèn in Mandarin and has kan'on jin. *ɻiC > ɻəC? Could be, since the apical vowel can't occur with a coda consonant.)

(edit: I should probably point out that I don't actually know most of this stuff -- I just know how to look it up. So my sources could be wrong or I could be misinterpreting.)