The marketcap goes up, because every 2.5 minutes there are 12.5 coins added to the total supply.  Each of these coins are valued at .0055BTC, which means someone has to buy, hold, or dump them.  Every day that equals about $30k USD that needs to be pumped into the ecosystem for the price to hold, which is about 40btc.  This is how the price can go down and the market cap moves up.  If this pace continues, this will be a top 40 coin in a month, and way higher in 2 months.

You'll notice on the exchanges the volume is usually around 40btc, if it goes below that threshold, the price will plummet, because most miners dump everything they mine.

The question is, how long can the buyers keep buying before they have to make a move.  Will they just just dump and recoup some money or double down and try to move up the price?  You would have to be pretty rich to buy 40btc a day to maintain the price until POW halves.