As a very longstanding Guildmaster (the fact my guild is based in Parrius is irrelevant in this argument) I have a couple of points to make regarding new guilds.
Of course they have to get their members from somewhere, however there is the consideration that the opportunities are there for the new guild to take experienced, equipped and trained up members from the other existing guilds.
Whilst no constraints are put on my guildmembers, it is always pointed out that although they can live in any city they want or follow any patron they want there must be a degree of loyalty to the guild.
The obvious potential revruits in the Artisans for any new SD Loremaster type guild have already accepted this fact and pledged their intent. I suggest that other GM's rather than let members either drift to new guilds or be actively recruited, discuss the situation and test loyalties at an early stage