Market cap is fixed and there's no way that people would be supporting an increase of the market cap. Doing so would just dilute the current Bitcoin and increase inflation. The fixed inflation is a core feature of Bitcoin and it is unlikely to change. Even with changing demand, a Bitcoin would be worth more and people can buy a fraction of a Bitcoin instead of the full Bitcoin. If Bitcoin ever gets too expensive, we'll probably be able to do a fork to introduce more denominations.