I can see a lot of praise of DCA here. I agree that it has its advantages: it's simple, affordable, it makes investing into Bitcoin a routine task, and overall it allows accumulating more and more BTC. However, I'd say that if someone has low income and can barely make ends meet, there's probably no good way to invest in Bitcoin. Because I firmly believe that people should only invest what they can afford to lose. Also, DCA is good overall primarily because Bitcoin tends to have long bear markets, so it means that a person is mainly accumulating BTC at a low price (compared to ATH). But we don't know if that's going to be the case in the future, and if the price remains stably high or is even growing, maybe DCA isn't very good because one's now getting a good price for it.
Of course any long term investment such as 4-10 years or longer should be taken from disposable income so that means that it is money that you do not need for short term expenses and perhaps even for medium to long term expenses, since as you mentioned that it is money that you can afford to lose so that you are ready, willing and able to lose 100% of it.. even though no one invests into something they expect will go to zero, even with a great investment such as bitcoin (perhaps the best investment currently available), there are still downside scenarios and even possibilities of it going to zero.
Another thing if you are investing long term and using a DCA approach, there might not be any exact idea about expected price specifics in the future or the exact timeline or circumstances of the investment beyond perhaps having some general parameters in regards to time and/or price in the future, yet there should be a presumption that the price trajectory is more likely to be up rather than down.. otherwise you would not want to invest into it.
Yeah, you might end up being wrong, but you would still figure out whether to invest based on your expectations that the price trajectory is more likely to be up than down. With a long term investment DCA works since it is not necessarily based on pump and dump ideas, yet people do also use DCA for pump and dump and trading kind of scenarios too, even though they may well expect to be in the asset for a shorter period of time than a long term investor and might even pre-establish some more specific price and/or time targets, which may well not be the case with a longer term investors - even though surely individuals have their own ways of establishing their price expectations and their timelines and whether they consider the investment to be long, medium or short. .so the categories of investing versus trading can surely overlap.... and the applicability of DCA is more based on how to get into the investment rather than needing to specifically establish how or when to get out of the asset... .