Investing in bitcoin is not always about the amount but how we can be consistent in investing.
This is a stupid take, consistency in investing also results in a final amount, and even DCA would of course be influenced by the amount poured in, Do you think DCA with $20 a day would beat somebody buying 10 million now?
You will need 14 years of constant DCA at those sums and a 10 times better average buy price to beat the profits of a guy buying coins right now, so good luck with that!
It boils down to what a person would like to reach - sure, I do agree with your take that if you invest once in BTC for $10M, it would be more profitable in the long run, however, not everybody has such and even lower capital for that
People need to start from something, and doing it in a lump is not a good decision overall, but not a bad one if we are talking about Bitcoin.
DCA is for your investment to not suffer that much from volatility, however, some people do it in lumps - and they are too in profit, though I didn't compare the two seriously. As you said, it depends on the volume you are willing to use.