you could actually avoid this conclusion by assuming that painful experiences of a given intensity have diminishing marginal value and that this value converges to a finite quantity.

Yeah, this is a common attempt to avoid this particular repugnant conclusion. This approach leads to conclusions like that a 3^^^3 mildly stabbed toes are better than a single moderately stabbed one. (Because if not, we can construct an unbroken chain of comparable pain experiences from specks to torture.)

However, I see no independent motivation for embracing a theory of value of this sort.

The motivation is there, to make dust specks and torture incomparable. Unfortunately, this approach doesn't work, as it results in infinitely many arbitrarily defined discontinuities.