I think having the founders own a total of 50% is a seemingly high risk but that is totally opinion based. If the other entire 50% rise up in agreement could they halt the company? Wouldn't 60-40 in favor of the founders give them a little more leverage because after all it is their shared idea/vision/etc and they should be shouldering a large part of the risk.
If you're ever in a situation where the entire remainder of the company, investors included, are wanting the company to go in a completely different direction then I would have to agree that something is terribly wrong. Maybe 50% is enough. Maybe I'm bringing up a hypothetical that has no basis in reality.
If you're especially worried about it, you can pick one co-founder (perhaps the CEO) and give him +1 share so the money is still relatively even but the cofounders can outvote the rest. In practice I think once you have a board, it's more important to control the board than worry about base share distribution.
If you're ever in a situation where the entire remainder of the company, investors included, are wanting the company to go in a completely different direction then I would have to agree that something is terribly wrong. Maybe 50% is enough. Maybe I'm bringing up a hypothetical that has no basis in reality.