I really hate how this is argued, not only by you but every time I see this brought up. Some of the result based examples you give are clearly not "the best team" even if they got results. The easiest example you use that should never be on any list that is "best team" is C9 2018. This is arguably the best result NA have in recent years depending on how one value msi 2019.
Its hard to account for circumstances for every result. Arguably g2 should have lost MSI to IG. But don't counter point look at this result when LS says best performance. "Every" year we have post that 1st team NA got screwed in groups and to be frank there is some truth to this. Often getting 2/3rd place LCK/LPL is not an advantage (contra having just 1st place LCK/LPL).
Its not hard to imagine a game looking looking close with SKT when they are on top is "better play" than beating Afreeca. But this will never show on W/L record. I don't think I agree this C9 is the best team NA have sent ever(?). There is to much small stuff, but in parts of the game they are competitive where they often got blasted (even if they still get snowballed from the the start) And RNG and DK came in as clear nr 1 from their region and that should be valued (mad too, but I guess I'm to used to G2 showing up to accept that).
Examples:
msi 2019 is a upset everyone knows this. But its an upset against the best, or one of the best teams in the world at the time. This is a great result no hiding it. However IG didn't preform, they didn't beat a great team on the day. Also the finals and groups showed that TL was not up to par.
TLDR next paragraph: Afreeca was not a top 3 LCK team summer/fall 2018.
Lets look at worlds 2018. If you followed LCK at the time you know that Afreeca had a stronger spring split however fell of in summer. However due to Griffin falling short to KT, Afreeca go to world on points as the 4th place team in LCK summer. Griffin, LCKs 2nd best team by far is not at world (they had just joined the league so couldn't have points from spring) did fall short in 2 times, to KT and Gen.g in qualifiers.
I think it should be easy to argue TL worlds 2018 was a better team than C9, at least to the point that it should be on your list of examples of good performances. But the list is good results and therefor c9 is there.
TLDR: Hard to account for circumstances, but results are not equal good (relative to the time) gameplay. LCS 2 best results in Worlds 2018 and MSI 2019 are generally considered upsets that are far from "peak gameplay" at the time. In the old days it was how far away you were from LCK and SKT, but now that best team change around more and I guess its also harder to gauge how far away you are.