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And this is bad design.
If want to replace a unit with a timing tax or build it at a point in the game past its prime, then I'm being punished just because a unit could have potentially come early and created shock value - that's just silly.
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Not necessarily.
Generally speaking getting the price right for units is not very easy. But there are simple solutions for this issue.
For instance one could offer a discounts to lower tech vehicles once higher tech levels are achieved.