Sure, I understand that. But should I (in this specific example) experience any decay at all? The explanation for practical decay is that you need to keep building panzers in order to get practical experience with panzer technology and keep your research rate up; the US can't just spend the '30s researching carrier techs while keeping naval production at zero, then suddenly drop incredibly advanced carriers into the water without going through the prototype phase. That makes sense.
But for anything smaller than a carrier, you don't necessarily have to build a whole new division in order to do prototypes. The historical German army didn't say, "Oh, we're thinking about developing the Panther tank, and in order to do that we need to field an entirely new division composed entirely of Panthers." It gradually replaced the existing panzers in the field with Panthers. I don't see why (apart, maybe, from technical programming or game-balance considerations) deciding to create a new panzer division would give you a bonus to research, but deciding to replace the panzers in a current division with the latest model wouldn't grant such a bonus.
Basically, I'm arguing that spending IC on upgrades and reinforcements for panzer divisions should have the same bonus to research as IC spent building panzer divisions. I hope King or somebody takes a look at this and considers it. Maybe it's a bad idea for reasons I'm not seeing right now, but I would like them to reject it after giving it some consideration rather than have it not occur to them.