Nothing stops this at all. You can start your own right now, removing the 21M limit or adopting whatever rules you want, and mine as many coins as you like.
Likewise, you can sit down at your printer, and print lots of $100 bills with your own portrait, and invent whatever rules you want as to how they can be spent.
The question in both cases is whether anyone besides you will want to use them, and be willing to exchange them for goods or services. You'd have to convince people to use them, that your rules are such an improvement over the existing ones that they should switch. If they don't, then your new coins or bills have no real value.
So there's nothing stopping an infinite variety of altcoins from appearing; but probably only a few of them will be adopted by anyone.
Except that printing $100 bills with your own portait on them is illegal in most countries. :) – Jannes – 2013-12-06T13:50:35.473
Really, Jannes? How did the makers of Monopoly get away with just this (except for the portrait part), then? – pyramids – 2014-01-16T11:24:51.477