Blaming the Pioneer Plaque for the progressive degredation sounds like it makes sense at first, but the point of the Pioneer Plaque thing is that this Voldemort is supposed to be smarter than canon Voldemort, and a Pioneer Plaque horcrux superior. That theory makes the Pioneer Plaque horcrux inferior. Also I'm pretty sure Voldemort has other horcruxes, including Roger Bacon's diary and quite possibly ones hidden in the other locations Harry suggested when discussing how to get rid of a Dementor.