In practice it's hard to avoid having one, two, or three top players. The market tends to invite disruptors soon enough unless really underhanded things are done to keep them out, but in the meantime the situation can be unpleasant anyway. I'm not sure there is much of a fix, not least because it's difficult to define in law behaviors that are anti-competitive -- we all know them when we see them, but defining them ain't easy.