Connect the two dots(two extreme conditions) and draw a curve, high chances are that at last, your profits from all investments may not be that much, at least much less than what you expected when you first gained that $10K. Therefore, all the occasional profits between investments may become fixed or variable costs that these investment must pay.
Does this sound logical to you ? What do you think of all the interval losses from the investment gains ? Please let me know.
I don't fully understand your example. You invest 10k USD and turn it into 20k USD, which you are going to sell because you need the money. So basically you are down to 0 USD and can't invest directly in bitcoins again. If you don't need the full 20k USD the best option would be to only sell some of your coins and keep the rest of them. Also if you want to reinvest into bitcoins you don't need to put all your money in at a price of 40k USD. You could try and split out your purchases to get a better price over time. Then there is the issue that short term price drops doesn't mean we have a loss at our hand immediately, as long as we are not selling our coins we are not realising our loss and can recover a few month down the road again.