I think in general, it's probably best to avoid this kind of etymological explanation of the 'actual meaning' of some word or expression because they tend to obscure the usage and, well, actual actual meaning. Saying 'goodbye' in English doesn't really mean saying 'God be with you' to someone, even though that's what it's originally a contraction of. They're fine as etymology, of course!
When I say/hear "do svidanya", I usually don't consciously interpret it as "till next meeting" each time, it's a set phrase and it's usually perceived as a whole, "goodbye". Only you if you put some effort to pay attention to the actual roots that you realize, oh that actually means "till next meeting" (especially since in modern Russian, "svidanye", when used alone, now means "romantic date"). Same with "hello" which literally means "be healthy", it registers as just "hello" in my brain, I don't immediately think "they're wishing me good health". But maybe it's just me.
To a good first approximation, zero people mean "god be with you" when they say "goodbye". That might be the etymological origin of the word, but meanings shift over time.
Most people don't become polyglots either, though, so that's not a good measure of the utility. It's not a question of language-learners learning etymology would be useful to the man on the Chatham bus, it's a question of whether it is useful to language-learners.
Knowing the etymology of, e.g., "goodbye" makes it click faster/not be weird when you learn, e.g, that "hello" is "Dia is Muire duit" (God and Mary be with you) in Irish Gaelic.
because forgetting things about the only tool we have for communication, imperfect though it is, is always an improvement. How else will we rediscover them badly?
On the contrary, I find etymological explanations often most enlightening. Although, I am fully aware that it might just be an origin of a word or a phrase and not the contemporary meaning. Your argument sounds defeatist to me. To paraphrase it: since the 'actual meaning' alone is not a perfect explanation we should not even try to understand the roots of words and their heritage. Then you can stop understanding the world altogether, because no amount of knowledge will remove all obscurity and contradictions. Yet human knowledge is prospering.
I concur, one can learn a lot about the culture when learning a new language. For example, in Swedish you say hello simply with “hej”. And as you may have guessed, Swedes are not big on introductory formalities even in the business context. On the other hand, German and many other languages have different words for you and You (“du” and “Sie”, pronounced Zie). As one may guess, You do not address unfamiliar people with “du”, there is a process to get to know people to the point when You ask a person if you shall switch to a “first-name basis” (this phrase gives a hint that in English, one uses the last name to address an unfamiliar person instead).
Edit: I was deeply impressed when I saw a photo of one of Einstein’s letters after he migrated to the US. Such a highly accomplished scientist still opened his letters to colleagues with “Sehr geehrter Herr Professor Dr.” (highly esteemed dr. professor, sir). This tells me all I need to know how cultured Einstein was.
