It seems to me that the only solution would be for bitcoins to be perpetually made available (no limit at 21 million).
There are altcoins which follow that pattern, and some have a significantly larger sum.
Of course, Bitcoin wouldn't be Bitcoin if more than 21 million could be mined.
What you have to remember is that there are potentially 2.1e+13 units available. At this rate a single coin will be enough to buy a house, people are turning to mBTC and such.
Transaction speeds need to increase, more off-chain activity is needed, transaction fees are higher than ever (in terms of $ value) - that's a huge blocker for many...