ToE does not operate from the presupposition that variation is open-ended. Rather it does not assume limits without evidence. So your objection here is that evolutionary theory does not accept assumptions made for the primary purpose of denying evolutionary theory.
As for your comments on "genetic capacity" it appears that it is an ad hoc premise with no supporting evidence. It may be the most reasonable position IF you assume a YEC framework and your claims of ever-decreasing diversity but it is not a reasonable position. If we don't assume a YEC framework (against the evidence) we don't need to explain why the expected evidence is absent.
For a start it is clear that the "original kinds" had no need for all this supposed capacity - so why would they be given it ? Moreover there is no sign of it in even the oldest DNA samples found. In a YEC chronology it has to survive ~2000 years leading up to the flood and then rapidly disappear leaving no identifiable trace. Not even in animals that YECs would say lived a relatively short time after the Flood (Mammoths).