You are very correct. It's a wise decision for a person that have a lump sum amount to accumulate Bitcoin as much as possible without delay because Bitcoin is very volatile and while delaying and still contemplating on DCA or waiting for the price to dip before buying the price may skyrocket beyond the person budget.
Surely I agree with you that if a person has money to lump sum once at go, he should invest it fast as possible without hesitation but that does not mean that Bitcoin will outgrown the impossibility of buying according to your Budget. Of course that is a misinformation Bitcoin nature permit any one to buy according to their budget. For example if 1 Bitcoin today is $100k and you have $100k to buy 1 Bitcoin but due to procrastinatination you somehow waited for Bitcoin to increase to $120k surely you will not be able to buy 1 Bitcoin $100k, you will ber able to afford 0.833
BTC with $100k and not 1 or 0.99
BTC. So surely Bitcoin is affordable with any amount you have but not less than $5.