"Consider now the subsequences starting at the smallest natural number: inclusion of the upper bound would then force the latter to be unnatural by the time the sequence has shrunk to the empty one."

Maybe it is too late, but I don't understand this sentence.

A sequence starting at the smallest natural number would be: 0, 1, 2. The variant Dijkstra is arguing about is: 0 <= i <= 2. How can the latter be unnatural? And what does he mean with "shrunk"?

    [0,1) = (0)
    [0,0) = ()
    [0,0] = (0)
    [0,x] = ()

Probably you'd say x=-1 but if you're using unsigned indices, then you can't distinguish the biggest possible sequence from the empty one (this is true of all schemes, actually, but at least the other don't require negative numbers).