21M is an odd number for a coin. And it is broken into such small units (.00000001 BTC = 1 satoshi). I think Bitcoin is already the world's highest-valued currency unit (
http://en.wikipedia.org/wiki/Highest-valued_currency_unit). The 2nd highest is the Kuwaiti dinar, which is worth $3.51. Bitcoins are currently trading for $156.82. To me, this seems to give it an excessive amount of transactions in fractional value, and seems to be a weakness of the original design. I suspect that the inventor, Satoshi Nakamoto, simply didn't expect it to become as popular as it has, or didn't pay much attention to this aspect of the design. Although I suppose the flaw isn't too terrible, since things can always be priced in millicoins (0.001 BTC) or microcoins (0.000001 BTC).
As to its increasing usage, that is unknown and speculative. As a technology, Bitcoin could always be replaced by something better that consumes less energy or has more official support. There are already so many Bitcoin clones crowding the market that it may become fragmented like Linux, hindering its wider adoption. The major push for recent usage of Bitcoin seems to have come from Europe where they are having ongoing banking crises, thus many have learned that their money invested in Euros is not entirely safe. There is no guarantee that this sort of interest will continue, though. I don't think the average member of the investing public even understands Bitcoin or how to value it.
There is at present no plan that I am aware of to replace lost coins. The problem is that there is no way to distinguish a "lost coin" from a coin that is being saved by someone for later spending. On the other hand, with each lost coin, the other coins become more valuable, and they can always subdivide the coins further to extract more value from them. Some economists think that this is a flaw in the design, also, because the coins are deflationary, meaning they grow in value over time, which encourages hoarding of the coins and reduces the available of currency to be used for spending. If you don't like these qualities, there are altcoins (alternative coins) that have some rate of inflation built in, which would help with these issues.