Is there anything to stop the core devs saying we want unlimited bitcoin and changing the code to allow it. I mean, 21 million isn't much.

You a Sr and making posts like this? -_-
The number of coins doesn't matter. Morecoins = More divisions thus less price Lesscoins = Less decisions thus higher price.
You know how a pizza is cut? Then you know why your questions is ridiculous.
1 Pizza is still 1 pizza no matter how many times you cut it.