Also if you only have bitcoin, then it depends upon you if you want to buy some gold too, I would say to keep your portfolio in bitcoin but later when bitcoin reaches new All time high, convert some of that bitcoin into gold.
This is not really that bad of a suggestion.. and there might even be some practicality to it.. yet it also might be where part of the contention lies, and maybe we need to be more specific about this hypothetical person who might believe it to be a good idea to sell off some bitcoin to buy gold rather than buying something else or even is there a need to sell bitcoin in the first place. how long has the hypothetical person been accumulating his bitcoin?
Therefore, how big is such person's BTC holdings in comparison to other investment assets that s/he owns?.. and then gosh, maybe we need to get into the other individual particulars that relate to cashflow, timeline, risk tolerance and the various others.
Another angle is what you might be considering ATH? Start selling BTC at $69,001? or you are thinking about some other price point to start selling some bitcoin for gold... and then how much? and what if the BTC price keeps going up and I am selling BTC in increments, might I run out of BTC at some point?
Then once we are holding some gold, what do we do with it? Are you thinking physical gold or paper derivatives? Oh my the more I think about this idea of selling bitcoin for gold, seems like a BIG ass waste of time and efforts and probably money too.... especially if you consider what purpose the gold is supposed to serve in my portfolio as compared to some other asset/currency, that might include cash, property, equities, bonds or even other commodities besides gold.