what makes you think that's what most people do? on the contrary, i think most people sell the bottom and/or buy the top. they get greedy when price is rising and don't want to sell. that's why the old warren buffet saying always applies. 
that is the logical move for anyone who is not a hardcore trader who cares only about making money. of course there is the buy high sell low situation but that doesn't mean people aren't going to spend 0.001-0.01BTC of what they have.
also and mostly i am talking based on BitPay stats. for example this chart from 2 years ago was the best thing i can show you. it maps number of processed transactions by BitPay (the bars) versus the price (the line). it shows an increase number of them during the ATH of 2013 and a big drop after price came down and then slow growth afterwards
this is the latest stats which is describing the same thing with 328% rise but it doesn't have the same pretty chart!