I would say it's more like a café keeping a type of cookie on the menu, even if it's similar to other cookies, and even though shelf space etc limits the number of different types of cookies. Is it a good idea or a bad idea? I'd say it depends on whether or not enough people like having access to that type of cookie. And the same applies to the factions in the game, do enough people like to have that variety in the Hellenistic factions? And is it possible for them to be different enough to make them interesting to play? I don't know, but as far as I can tell people do appreciate the variety they add to the Hellenes.
If anything it's the opposite I'd say, it's during the Alpha phase we can afford to be flexible. That said I would say that it is (i.e. at the moment, and depending on the people in the team and current opinions) the team's decision not to add any more factions for part one at all. But as with everything else it could change at a later date.
I do however think that it is a good idea to be very strict with changing things on this level, especially since we are getting close to Beta. If we remove factions people are going to get disappointed, work that has already been put into them will be wasted, and since all limits are at least somewhat arbitrary there is some merit to staying with the ones we already have decided upon. Otherwise where do we draw the line?
There has been plenty of arguments as to why the Iberians should be split up into at least two factions if not more. At the very least the author of Delenda Est has argued for adding another Hellenistic faction: the Thebans, and I do recall someone else arguing for having some other Hellenistic factions as well.
There are plenty of other factions which could be added, of them it's probably the Scythians which are the most interesting imho. It is impossible to make a perfect decision though (there will always be someone who is disappointed, and arguments as to why you should have made a different decision), so the question is where do you draw the line and say "these are the factions we are going to include". I think it's better to have the focus gained from not having to consider the question again, and again, even if it does mean that we might be missing out on some interesting factions.