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that is true, but also, you have the total research hours a topic takes to research in the game.
and usually, that is not a hundred scientists researching for a hundred years.
and believe me, i can easily produce research ideas that can make the research side of the game insanely complicated and research topics intertwined, and research time somewhat random, and...
...and have multiple results from researching the same alien item interrelated with other researches.
but as far as my opinion goes, this is a tactical combat game rather than a research game.
so, diminishing returns on research would just give some meaning to number of scientists assigned to a specific topic, and make it a strategic decision.
otherwise, all scientists in a base should always be working on one topic only.
i mean, without diminishing returns, it should never make sense to assign scientists to multiple topics at the same time, unless you deliberately wanted to handicap yourself.