Selling and taking profit during the bull run is not ideal because after selling, you might not be able to buy back at the price that you bought before and this will make you have less bitcoin that the initial amount that you had before selling. Meaning that you don't have a bitcoin target and you are not focus on accumulating bitcoin on a long term investment. Another reason why this strategy is not the best is that you will miss out the compounding interest that your bitcoin investment will accumulate for you if you only keep on accumulating till 10yrs time or more. Try and see your bitcoin investment as your life savings and not for profit alone because nobody knows if at the long run it is your bitcoin investment will be the one to take your to the next level in future because there is high possibility that bitcoin price might keep on increasing with timeline. It is better to save for the future than saving to sell and take profit and buy back again. All bitcoin sold can not be bought back easily because of the change in price towards the upside. It is also a bad idea to sell all your bitcoin investment and hold fiat. What is things didn't work out as you planned it after selling, that is when you will regret your action.
I was having this mindset before, thinking that it was never a good practice to sell Bitcoin. But when I join this discussion, I realized there were actually people that sold around the peak of 2021 bull run and started building back their portfolio during the following bear market even up this moment many of them are still buying. How they got such information about the market cycle and how they developed such level of discipline to follow it is what one need to study. The key point is that, there should be a way of selling that will not harm the entire portfolio especially when you sell not because you want to use the money for other things or waste it away but because you are being strategic and poised to maximizing profits.
On a second thought, selling does not harm Bitcoin all the time because without buying and selling, what is the use of Bitcoin? Indeed, it is through selling and buying that liquidity is created so we should not demonize selling. Just that one should be be able to plan the selling in such a way that it align with your visions and your motivation for investing in Bitcoin in the first place.