When is a research question "closed"?
I don't think a research question is every “closed”, as you say, though it's of course a matter of vocabulary. In the example you mention, it seems clearly that there is no current incentive to design better solutions, but unless it is actually proven that there can be none, it's not a solved-and-closed question, it's a “we don't actually need to do better” question. This makes all the difference in the world.