Even strong hands get weaker over time. Gravity.
I suppose your comment has merit. But I think that it isn't a completely proportional concept to price rising.
When the prices rebound and remain stable strong hands get stronger. As they can expect to know the price is going to rebound as it has so many times before. Hence the load the truck with more bitcoins even though the price has risen over 1000% in a year.
This. Also, I believe that hands are getting stronger at a faster pace than that of the inflow of new coins and new selling pressure on the market. The dive from 1200$ was a huge workout for many, a considerable number of shaky hands have probably matured and grown stronger/more confident by now.