The idea of selling 50% and keeping 50% is also a good idea because we will not be able to predict what will be the highest increase of bitcoin during the bull season. But if you have set your sell target and the bitcoins have reached the sell target then you can also sell all your bitcoins. Sometimes we should not be too greedy because our greed can easily pay the price. Many investors in 2021 made the same mistake when bitcoin hit $69k but they wanted more and in the end they paid the price for their greed.
Yeah you have a point but perhaps it may not even be greedy for them not to have sold there Bitcoin when the price was $69k because irrespective of the profit they may have gotten then but perhaps there intentions may have been just hold for a very long and never to be influenced by the price movement because unlike most people that invest and keep watching the Bitcoin price to sell if it comes up, however there also who has a lot of money and decided to invest most of it on Bitcoin for twenty years so somebody like that should not be expected to have sold when the price was up.
So however in another perspective just like you said if they most sold is adviceable to sell 50% or less of your accumulated Bitcoin so that whatever happens you may not completely lose out, the other 50% will be your back up plans so that which ever way the market turns you will not be affected that much.