The number of BTC available to make large walls decreases as we go up, however:
In some sense this is true, that is, if we are only considering miners as the primary source of such large blocks. However, there are those that bought BTC at lower prices that might want to sell them at the seemingly ever higher and higher prices.
So, one such person/entity or many people/entities all converging on a common sell point (e.g. a multiple of 100) could muster up 15k of bitcoins to sell at those psychologically motivating levels. No matter how you cut it, there are 11 million bitcoins currently out there that can be traded. Most all of them at this moment in time are in the hands of people that acquired them at lower prices.
Admittedly, it will seem a relief - even if only justified as psychologically - to think miners have distributed the majority of their outstanding huge blocks of their coins.