The point of diminishing return will probably also depend on available technologies in a country and on strategic considerations and will be lower for China than for US.
Can you elaborate on that? I doubt it and strongly question the assertion. I suppose we're talking about construction and operation cost and efficiency, and battle field effectiveness. How is the optimal tradeoff point different between China and the US?
By the way, I always question the notion that all China aims for is 4 - 6 carriers and anything beyond is excessive and China has no need for them, unlike the US. I don't think the Chinese leadership and navy planners are seriously thinking anything beyond six carriers now since that's still far into the future, but to rule out the needs and possibilities of more than six carriers in the longer term (beyond 2030) is irresponsible.
Most NATO nations build their armament to complement the US, not to compete with; outside NATO, most nations can not even hope for nor can afford it to build to the same scale.
China stands on its own, has increasingly the national interest and strategic needs to develop the military capabilities that go with it. It will also have the resources to fund them. It will take time, for sure.
Fortunately, the Chinese leadership and PLA leadership have the right vision. Even at this early stage, we're seeing China plans to build a 12,000 ton class destroyer (055), not limiting it to under 10,000 tons just because Burke is; we're also seeing the 901 class supply ship under construction likely to exceed 50,000 tons; we've also heard and are seeing signs that China's first LHD will be around 40,000 tons, far exceeding all other nations and approaching the Wasp-class.
At this point, China still has considerable technology gaps and may not need the same quantity as the US in many armament, but that doesn't mean China aims or should aim for less quality.