DCAing don't really matter if the market is high or low, so you don't have to wait till the market value goes down before investing. I agree with you about having an additional income or having a major source of income if you want to apply the DCA method, however making one big investment too is another strategy which is mostly the long sum method, which for me I believe is for people that have the purchasing powers to do so, however the whole idea is to best use a strategy that suits your income.
Yes those who invest in the DCA method have no impact on their investment even if the price of Bitcoin falls or rises because they buy Bitcoins with a fixed amount at a fixed time.But for doing DCA, everyone needs to have a specific plan so that he can keep his money invested in Bitcoin according to his ability according to that plan. However, having an additional income opportunity to invest in the DCA method is beneficial for us because we can invest the money in Bitcoins without needing our earned money.