So, despite my being willing to grant that there will be a meaningful market value for bitcoin far into the future, and that the things described above might take decades to unfold, the simple observation that it's a purely capital asset leads you to realize, by the reasoning above, that demand WILL top out, and with it the price, which will almost certainly then start falling as its new long term trend.

This might be true, I don't know, but it doesn't seem to have happened yet to gold, also a capital asset with no earnings or coupons, which has been around a long time. Monet's Meules sold for a record $110.7 million five years ago, double its pre-estimated value of $55 million, and he's been dead nearly a hundred years. Maybe the durability of demand depends on the quality of the asset.

A sated world population suggests a static world population, which, of course, is never the case. There is always a new generation coming along, providing fresh demand for many things.

In any case, even if it is true, if the main threat to Bitcoin's price is sated world demand, I would guess, as you say, that it's far off in the future. Corporate, institutional and sovereign demand are just now ramping up. The demand curve looks to be at a relatively early stage.