Bitcoins are very divisible. As more and more people use Bitcoins, the coins become worth more and people can simply trade smaller and smaller pieces of them. The limited supply doesn't limit the number of users.
EDIT: answered my own question
Not true.
> 21 million is arbitrary and meaningless.
Also not true.
Changing either of those things requires a hard fork of the network and a majority consensus among the miners globally which is in no way "easy to do".
Disagree with that all you want. My point is that it's possible. Speculating about the likelihood of it succeeding doesn't seem worthwhile to me.