That tends to be one of the additional problems when traders are failing/refusing to sufficiently/adequately account for a certain kind of exponential upwards price pressure that exist in bitcoin, so they frequently will either consider the price dynamics of bitcoin as if it were a kind of mature asset or that bitcoin is correlated with mature assets or that everything that goes up must come down and other kinds of insufficient frameworks to consider BTC's price dynamics and potentials that end up costing them a lot of money for not sufficiently/adequately account for such, even when they might have had been successful in their execution of trades for many repeated times, it might ONLY take one or two wrong calls to either completely or mostly wipe them out of both their previous profits and then potentially sufficiently reduce their principle too.
IMO this is a clear reason why anyone who wants to accumulate bitcoin for long term should stay away from any scheme that has something to do with selling and if you look at the risk involved and we were to give a possibility of the outcome that buying back would be possible then we could be stuck there because there are too many uncertainties involved and no-one can really tell if that risk even has a 0% possibility of happening( my reason beign that just as you said they forget that bitcoin is still an early asset and has more possibility of going even higher and never returning to old prices and even so bitcoinis a very unpredictable asset), then at this point it is more like a gamble and why would anyone who really has good plans to buy more bitcoin consider this as part of his strategy.
Besides if you really wanted to accumulate more bitcoin then you should be more concerned with looking for means to increase our finances to be able able to buy more bitcoin( this could include having multiple income streams or finding ways to move some value we have in other asset to bitcoin just like the last example you gave where the guy has a non taxable event and is able to move up to 12k into bitcoin) which clearly has less risk(would possible pay out better)involved in it than selling to buy back unless anyone who wants to do this would be ready to accept the reality that he bitcoin price might never come back low again and he can end up buying back at an even higher rate than he sold, so this should be one of the senerios in his possible outcome and then if he still wants to proceed with the gamble, it's clearly his choice.
I do agree with you on this, another disadvantage of engaging in this kind of practices too early is the risk of getting stuck in such ways and can potentially wreak your mindset and slow down your accumulation process even more than you had intended when you felt that buying back would give you more advantage to acquire more bitcoin, and eventually you end up selling and bitcoin continues to rise and never goes low again , you are either left with two options to quit or to buy back higher and I believe that so many people that sold off to much of their bitcoin too early are in much regret right now.
I wonder if we could put this in a fair hypothetical to further explain this possibility properly.