Maybe OP should have slowed down with anticipation on what the future hold for us as it has to do with Bitcoin because the crypto space is still an evolving one and between now and 2140 is a very long time enough for a whole lot to happen both with advancement and revolutionary changes in the space. Whatever we may say now about the future of Bitcoin are mostly speculative because no one has an exact ideas on with it would be else they will just go and hijack the whole system.

Even after all the Bitcoin has been minned, provided it is still traded, it will still be possible to DCA Bitcoin and accumulate as much as you want over the period of time you can. Not everyone at that time will want to just hold on to their Bitcoin,  because making money from it will entail that you sell off or a fragment of it after the value has appreciated before it becomes money you can have in your account to be spent on things you wish to spend it on and at this point you will sell to someone who wants to buy and if you don't have as much quantity the person will keep gathering around till they get the much they want using DCA while buying from all of you.