Why do we use plural forms in wishes with -ler and -lar?
Like -- Iyi günler ? Iyi akşamlar etc.. ?
My extremely farfetched, totally unsupported idea… Maybe the intended meaning is: "In case something happens and we don't ever see each other again, I hope you will good nightS in the rest of your life," or something like that.
Note that we're not the only ones that are weird like that: Spanish also uses plural in wishes. Buenas noches anyone?. ;p
As far as I researched on internet over the Spanish example, it's believed to be due to the fact that in those times people had limited means of communications so they saw each other rarely. so instead of saying good night for one night, they expressed this wish for several nights during which they wouldn't see each other. This one is similar to your theory.
Another theory is that it's related to the polite form of noun cases which are conjugated accordingly in Latin, so the plurality adds a sense of formalness and politeness. But this cannot be case for Turkish. Maybe we both borrowed it from Arabic