Holding really matters in investing in Bitcoin but it doesn't mean we have to do this our whole life. If we take a look at the price chart, prices are growing but can we think this will be the same trend in the next few years? Not for sure and it was uncertain. So I could say that holding will not be forever as it also depends on the market situation. Selling our Bitcoin during the bull season is a perfect option IMHO and get on the train back again once the market correction starts. But yes, this needs strong hands because waiting for 4 years is not as easy as people think as we need to deal our emotions as well.
People have different reasons and target while investing in Bitcoin with this target determining, to a large extent, how they will going about selling and managing their portfolio. While majority of Bitcoin holders are looking at the profits they can possibly make from Bitcoin, a good number of Bitcoin investors including myself, already made up our mind to HODL Bitcoin as long as we live and even pass same to the next generation. Under this premise, selling during bull run is already out of the picture even though some could consider liquidating a portion of their Bitcoin stash with the aim of reinvesting same during bear season (this being just an option some people might consider while some will sell only when they want to pull out funds for other tangible projects that their emergency funds cannot cover).
So the pattern of selling off one's Bitcoin portfolio during bull market with the hope of buying them back during bear season may be good but not for everyone. In the end, it boil down to individual preference just like I explained before.