Even though I agree with both your math (above), and I agree with the theoretical point that any investor is going to be able to take out all of his/her principle at some point in which BTC price triples (in your example) or goes up higher, and theoretically still be left with a cost basis of either zero or at least theoretically proclaim that s/he is just playing with house money because s/he has taken out the value of the principle that had been invested.

I personally do not have any problems with strategizing and planning to regularly aggressively withdraw various amounts of BTC as BTC is in profits - however, personally, to me, it continues to seem to be relatively short-sighted thinking that there is some kind of a preference to get back your principle and to get your cost basis in bitcoin down to zero - especially problematic when BTC prices may have merely appreciated 3x or so the amount of your investment - and it surely seems to me that within a longer timeframe bitcoin still remains the best of asset classes, and therefore, there should be little to no general motive to diversify out of it so rapidly - even if there are no guarantees of BTC price appreciation and even though we sometimes experience some pretty serious price corrections (relatively speaking). 

Maybe we can consider a more modest hypothetical of someone who might have bought $100k worth of bitcoin in late 2013, so s/he made some mistakes and bought near the top of the market at about $1k per BTC, so that person had gotten 100 BTC, and had to ride out the ups and downs of 2014 through 2016, so finally getting back into 2017, s/he is getting back into profits, and when BTC prices reach $3k in 2017, s/he decides to get back his her principle, so sells 33.3 BTC at  around $3k.  Therefore, s/he is left with 66.7 BTC rather than 100 BTC.  Yeah, we can see where this goes. 

I will admit that such above hypothetical person is way better off with 66.7 BTC than s/he would have been if s/he had not invested into BTC, but it also seems to me that figuring out a way to hold onto more of those 100 BTC (rather than getting so worked up about ensuring to get back the principle) would have likely been a better overall approach to managing the HODLings... and I am not even saying that individual circumstances might not dictate getting back the principle, but it also seems to me that if there is so much anxiety about getting back the principle, then a lot of mistakes had already been made in terms of both investing near the top and then spending about 3 years in the negative without taking further buying actions that would have likely lessened anxiety and been more practical overall in terms of managing the bitcoin investment. 

Of course, even if we are assuming a kind of lump sum investing near the top and a further failure to continue to invest in BTC for the next three years that BTC prices were lower than the first cost of investing, the more prudent approach still seems to be a kind of skimming of BTC along the way rather than some kind of a seeming urgency to get back principle, and a skimming approach may well have ended up reducing the BTC down to 80 or 90 BTC rather than 66.7 BTC.... and so it seems to me that each of us has to be careful in regards to getting too anxious about getting our principle back rather than attempting to better understand the investment that we are in... because even with Bitcoin prices largely bouncing between $30k and $69k in the past year-ish, there would have been a lot of opportunities, to merely sell a few bitcoins 2-3 BTC, and get the whole of the principle back - if there was such a desire to skim that much at any given time along the way and still not reduce the overall bitcoin holdings by very much).