need to be able to get into lower tiers after they've been closed out by new tech. game-breaking if you finished researching a new tech with unbalanced production in a lower tier.
i played through the stuff everyone else is complaining about and it's a great game-- this is the big problem later on.
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In general, items are "closing" if you opened tech which tier are more complicated by 2 units, so in most cases there no reason to use outdated items.