I love the way strat design works, I think it's an elegant and subtle way of making sure games are varied and also realistic.
Here's how I think of it:
The unit design doesn't represent just how good a weapon system is - the effectiveness of different formations has always been reliant on training, doctrine and logistical assumptions. When you design a unit your model design team isn't spending months worrying about "wait...could we give this new gun to a soldier???" A new design represents not just technical designs ("Automatic rifle technology) -> "Chobsky & Co Military Rifle 2092") but also the assumptions, tactics, support chain, that take you from ("Chobsky & Co Military Rifle 2092" -> "Piedtruppen '93")
When you redesign, you go from "Piedtruppen '93" to "Piedtruppen '98". Even without weapon changes, you apply lessons learnt - maybe they carry less ammo per soldier, because you find weight makes them less effective and your supply assumptions weren't tweaked quite right. But if "Piedtruppen '93" were a flawed formation because there was a bit too much emphasis on personal courage over discipline, squad sizes were too big to be maximally effective, and your supply chains assumed more coherence than exists on the battlefield so sometimes logistics bogged down on a divisional level, the Piedtruppen '98 are based on the same structural design choices.
You want different? That requires a structural redesign - *and your designers already did the best they could at first pass* So you want a new design, even for infantry formations, let alone armour? It'll take longer. And, probably, some folk need to lose their jobs, to get lead designers with different assumptions about doctrine, etc etc. Roll another unlucky str design? Sounds like next time through you need to sack most of your design bureau. They're all entrenched in a philosophy of design that just doesn't get the best results. Of course, at some point, you're going to have a ****ty tank design - just like some nations historically did.
By thinking about it as formation design - the whole way that a group of units fights, rather than individual tank/soldier design, it makes total sense to me.
As another angle to approach it, I'm pretty sure anyone at least as groggy as me can name militaries that have kept a reputation through a drastic evolution in the technology they fight with. From wooden ships with cannon to modern carriers in the royal navy, for instance. Or even if you insist on seeing it as a model only design process and for a less extreme example, how long has the structural design of the M1 Tank provided the US with a good MBT? It's 40 years old! Do you think the structural design of the original M1 doesn't play in to its effectiveness today? Of course not - the incremental improvements were cheaper than designing a new tank, and the initial structural design still affects the modern variants.