There are a lot of creative ways to calculate how much cash you have available now and account for your cashflow and also account for being prepared in case the BTC price goes down instead of up.. but sometimes people are so determined that the price is ulitmately gong up that they do not mind just front loading all of it and not preparing for down or sideways and their cashflow does not matter as much as it does to people who might rely more on cashflows rathe than money that they can move around from other investments.
And that to me is the brilliance of DCA which I will always recommend, in fact, the only thing I advice. Whatever the situation, whatever the financial cashflow available and whatever the risk appetite, DCA always wins you some gains even as short as 2 to 3 years (the entire bear window in a cycle).
It even gives confidence and experience to those who can't afford strategy or don't have wisdom. It's like a surefire way, I wish I did it much earlier.