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Unless that group of friends goes directly against the alleged design of "1 of each is optimal" - they'll just add number of people on the server rather than bring imbalance to the classes.
But even if we assume that it's not a full stack of people and they did go against that design - one (or even a few) group of friends is nothing in the face of the potential 10k players on the server.
But I think I get what you're saying, if people just choose servers w/o looking at the stats or caring about minmaxing their party (which will obviously be the case for quite a lot of people) - you'll get unbalanced servers. At least initially.
But I think that would be fine. Content will be mainly pushed by the minmaxers who will be in optimal stacks (so theoretically "1 of each"). All the other people will just be playing at their low-mid pace and probably doing solo or small group content. This setup would bring some imbalance to the archetype stats at release. But then, every new player who have chosen some archetype and doesn't have a friends group (of which there'll be a ton) can look for the server that has a fitting imbalance. And now this person can easily find a group to play with, because statistically his archetype would be needed there.
Obviously they'll have to put in work, because some parties might've gone for a quick alt switch to make their party optimal, but if you're leveling a main with the explicit desire to go deep into the game - I'd assume any party who's lacking a main of that archetype would be glad to get you. At least I know I would , if I was in that kind of situation.