Change to something new... which goes for every franchise... and the inevitable result is always the same. Some will leave, some will continue, and new ones will join. No idea if anyone actually has a definitive answer to that with a lot of non-subjective data to back it up. Simply personal experience, or the opinions of people you know, isnt enough to judge.
In any case, another question I suppose is whether or not there is such a thing as a 'main' server. It seems to suggest that players will go there regardless of whats on it because it has the IP. Perhaps, to remove OW2 from the question entirely... no argument about old vs new... ditch it and set up another G13. Will traffic remain the same because it is the 'main'? Or will they go elsewhere? Will the G13 crowd move because its the 'main' server?
Dont know myself, but does this 'main' server thing go for all servers? Including all games? Or is it one server among equals? My guess, go ask regulars there and they will say 'main' is bullshit. There is their own personal 'main' and that is it.