There are only max 21M bitcoins, so there can be only 21M people that have one btc.
So if it goes really mainstream it can go much higher and it's also a counterweight to fiat money.
The advantage of using a cryptocurrency like bitcoin is that you can divide the coin to millions divisions. The lowest fiat denomination is the 5 cents coin (0.05 unit). People don't have to carry 1
BTC exactly so bitcoins having a limitation of 21 million isn't a problem at all. The amount of bitcoins in circulation is probably much less than 21 millions though, but we'll never know of course.
"So if it goes really mainstream it can go much higher" - The supply wouldn't increase just because people are already using bitcoins. People simply have to settle about the fact that bitcoin is scarce and that's what gives it value (i.e. supposed that there's unlimited supply of bitcoin, then it will be valueless)