In my opinion, it is not a big mistake because, of course, selling part BTC and then buying it back when the price drops is possible, but we can only do that if we can accurately predict where the price will go. However, if we can accurately predict the price of BTC by doing that, then we will definitely increase our BTC holdings.
I believe that no matter the level of knowledge that you have in reading charts or in bitcoin investment, it's impossible to time the market which makes what you are saying not useful for a long-term bitcoin investor who has a bitcoin target that he is accumulating towards. Why do you think that traders run at loss in the long run because they cannot outsmart the market.
However, in this case, the decision lies with the BTC owner, as selling part of the BTC is acceptable as long as a profit has been made.
This is the problem that some bitcoin investors have, they are after profits in the beginning of their bitcoin investment which is a wrong mindset because if you put profits in your mind and you are on a long term journey, you will deviate from building your bitcoin portfolio persistently and consistently with ongoing buying without selling. Because whenever, you see little profit, you will rush to sell believing that you will buy back cheaper. You have deviated from been a long time investor and become a trader. Before you know it gradually, you will start depreciating your bitcoin portfolio by selling any time you like.
This becomes gambling practices because you have to start waiting after selling for the price of bitcoin to dip and if it doesn't dip, you will keep on waiting and might use the money for something else. You have finally lost focus on building and increasing your bitcoin investment for your future. Selling and buying back is not a good strategy to accumulate bitcoin with because you are trading. You will also depreciate your compounding value overtime and will end up with little bitcoin compared to buying without selling for 4-10 years and above