The crazy part if we are buying bitcoin for 4-10 years and even longer, we could end up buying at the top and at the ATH several times and still end up in profits over the long term with each of the purchases and/or with the overall average purchases, and we have no way to know if we are buying at the top, so we should not be thinking about those kinds of matters if we keep on accumulating bitcoin on a regular basis, as you mentioned.
If you are buying for last 10 years then you are not only buying at ATH but also at low price. ATH changes with time, like in 2017 ATH was 20k which today looks like an ordinary price. Today price is at it's peak of 115k and may be after few years from today 115k also looks to us an ordinary price. One advantage of keep buying for 10 years is that we don't need to worry about the bottom. Ten years averages the top, middle and bottom prices to give a good figure.