Most tech hits a point of diminishing returns.
I don't think we're there yet, but it's reasonable to expect at _some point_ your typical OS model could be 98% of the way to a cutting edge commercial model, and at that point your last sentence probably doesn't hold true.
There is cost/benefit analysis yes but fundamentally there are no diminishing returns because intelligence does not have diminishing returns.
One of the key concepts in the AI zeitgeist is the possibility of superintelligence. There will always be the possibility of a more productive AI agent.