Well, it's hard to compare since the format of League is so different.
Worlds and TI are very comparable, so lets equate those. Slightly in SKT's favor with 3 to 2.
I'd say 2 MSI titles are comparable to 2 Majors in terms of importance.
6 LCK titles, in my opinion, outweighs two majors.
The deciding factor for me is not necessarily the titles. When we look at esports in the modern era (2012~), we've never really seen a team like SKT. In CS we've seen Fnatic, NiP, Astralis and SK lead whole years, or even TWO years in the game's scene. In Dota 2, OG can be said to have been that for two years.
SKT isn't like that... they were the best overall team in 2013, 2015, 2016 and 2017. In 2019, they were definitely a top 3 team, bested only by G2 and FPX.
SKT's 2015-2017 ''dynasty'' can be considered equal to OG's or Astralis's, I won't deny those are equal. What sets them apart for me is the fact that they were also the very best in 2013 and are still, today, 6 years after their first success in League, one of the best teams in the world.
eague isn't Dota or CS. If there were as many international tournaments in League, SKT might have ended the year as the best team again. Unfortunately for them, they were bested in semis both times. Even as a G2 fan, I can say that SKT is the more consistent and probably better team.
Either way, that's why I consider them the best esports team of all time. They've had a dominant reign for 2-3 years just like Astralis and OG, but aside from that have been the very best at a different period in time and are still at the verge of returning to that again. 6 years is a very long time to be on top.