This is an insanely moot point when the average American isn’t going to know what Plopsaland de Panne, Phantasialand, or Efteling are.In Europe, Cedar Point is known, but the others don't even know what they are.
I’m really not sure what the argument you’re trying to make here is. If it’s that Six Flags is more well known, that is of course the reason they kept the name for the merged chain. That said, there are negative connotations with the Six Flags name, which many see as the budget chain. People may be unfamiliar with Cedar Fair as a name, but for example a resident of the Carolinas is likely going to know what Carowinds is, hell my boss knew exactly what I meant when I said I wanted a day off to go to Kings Dominion.