Raising the limit of 21 million BTC is not possible. (not suggesting that is what you're implying ..just that it is more than just not necessary -it's not an option) The only way to increase coin availability is to divide it into increasingly smaller fractions of a coin; .00000001 is currently the smallest unit, however, it can be divided into smaller units if demand justifies it.
1 Satoshi is nothing, not even spendable, so I dont see reason to divide Bitcoin more, not now, not in 20 years. But the theoretical possibility is there, I agree