Many people who has invested in bitcoin in the early days, at one point or the other made one mistake or the other and later realized themselves. Yes bitcoin has performed more than many people thought it would do in the early days. There were those who got bitcoin when it was less than $10 and sold when they saw $100 thinking it was the highest it could do. Many bought at $100 and sold at $1000, these are mistakes that were common with the early investors. Though the awareness in bitcoin today wasn't that much back then. So instead of dwelling on our mistakes which we can't do anything about, it's just best we get a fresh start and see how far we go, because bitcoin hasn't really achchived it's full potentials in terms of pricing.
I will not completely call selling bitcoin a mistake because if there were no sellers, there won't be buying and selling. Buying bitcoin and selling at $100 is not also categorically a mistake because that may be the target of the investors as his vision is limited to X10. People will continue to sell bitcoin based on their personal objectives which varies per individual, what I consider not proper is to assume that selling bitcoin generally constitute a mistake.
Investment in bitcoin generally have two phases namely: the accumulation phase and the liquidation phase. The accumulation phase involves building the portfolio and that is where we apply the various buying methods and how to be able to HODL the purchased bitcoin. beyond this accumulation phase is when it is expected that the investors should cash out some of his bitcoin asset at least to enjoy the dividend of his investment. This phase should also be followed systematically in order not to run out of bitcoin or end up regretting the manner in which it was done. I do not support selling too quickly because I see bitcoin beyond X10 as in the example you have given.