If you think about the teams that have been successful this year, it's all teams that kept (at least) 3 man cores together
Fnatic: Derke, Boaster, Alfajer
Loud: Aspas, Less, Saadhak
EG: Boostio, Jawgemo, C0m(Demon1 helped a lot but these 3 have been together for a long time)
NRG: FNS, Victor, Crashies
Paper Rex: kept original roster(Forsaken, Jingg, Davai, Mindfreak, and Benkai and added Something)
DRX(to a lesser extent): kept whole roster and added Foxy9
Even EDG(to a lesser extent): have been together ever since China first had pro teams(only the replacement of life)
All the best teams are teams that have stayed together mostly to keep chemistry and coordination strong
Even some Duo teams like Liquid(who kept Jamppi and Soulcas and added another duo:NAts and Redgar) have seen success because their structures were super strong