Is there some deep flaw in this assumption? Does the removal of the testicles result in a longer life expectancy fairly consistently?
To answer this question properly is going to need a prospect double blinded controlled trial. That is not going to happen in a normal world.
You can try looking at historical data, like the one referred to in this Huff Post article but that type of restrospective data is highly flawed.
their average non-eunuch contemporaries, who tended to live between 50.9 and 55.6 years.
So, the intact men were living short lives, much shorter than contemporary men suggesting that there was something in that ancient era that affected their health then. But what's the relevance to modern society?