Perhaps you should try to be specifying what you are saying ( category of investors that are not suppose to sell ) because investors who has gotten to there overaccumulation stage can sell if they wish to sell and nothing is wrong about that and even while they sell, they will always sell wisely that is a percentage that won't look like they even remove something from there portfolio so that covering up won't take them time but an investor with a small portfolio, selling will be a very bad decision or act to display because they have a long way to go.
Your statement might be misleading to those who have big portfolio that want to sell wisely. In as much as selling wisely should be considered, long term investors at times can make wrong decisions to sell when the market conditions are not in place. Technically i accept that timing the market is necessarily when we want to sell some fractions of our inestemnt. With a big size portofolio investors should understand that there must be a reason why you are selling if not there is no point selling yet when you can still keep it long more. The reasons should be solid, if the inteiton is to sell in order to hold FIAT it is not a wise move although we can do whatever we like as long as we have a big size portfolio and nothing could go wrong after all.