The way I see it, the ones who will cross over won't rush to level cap and instead enjoy the older content.
There was once a new German Server "Dochas". After a few days there were players from the other servers trying to rush through the contant ASAP and get all the serverfirsts etc. I think they made a guild full of people who played in the big guilds on other servers, not allowing any newcomers.
Edit: The guild(s) (and the server) died pretty quickly.
But I don't think that harms the server much. It's not like they would destroy the game for the others, just basically the same as if they were not there.
A little detail with what I remember from history:
For this server there was a global prize event competition for the first Annelia kill (- I think it was Annelia ... it certainly wasnt Kulech Bones Nest, and Bedim wasnt live yet) on the new server.
Which was the primary motivation of that amalgamated guild.
Just:
It didnt work out in due time because nobody wanted to do it at level let alone build and plus level gear for it when much better gear was available for FREE , and progress through questing was blocked - you couldn't get past Tergothen Bay because there was a bug in the main quest line in Dalanis.
Which as you might guess, that made leveling itself quite tedious since past Thunderhoof Hills most of the quests are gated behind the then non-working main quest.
(No main quest past Dalanis essentially means 90% less quests in any zone past that, and without completing the main quest, no trip from Tergothen to Ancient KIngdom of Rorazan and further)
Surprise, without a working main quest past level55 and therefore no higher dailies available dog meat value was weighed in diamonds. Literally multiple diamonds per SINGLE dog meat, not per stack. Either through its gold value or direct trade (which was still possible) or trade-gfiting.
(Cap 75 at the time, so ... powerquest, get lvl72 Eternal Circle weapons/equipment from NPC in Chrysalia, go)
The server was basically borked the first half year, except for those who spent loads of money just to be 75/75 early with nothing to do at all.
From what I recall, the "people from all over the high end guilds from other servers"-guild was merely a fun project and lost traction pretty fast when it became clear that the original goal of winning ther event prize would be either impossible or ridiculously expensive before the main quest was fixed.
Playing HoS at level again was fun though, been there done that.
/history
Dochas brought out high end guilds over time but in terms of first clears it lagged behind for quite a while of its existence (as in, it took them significantly longer than the other german servers).