The underlying mechanism of government pensions in their current state requires there to be more people paying in than being paid out, so growth is needed.

I don't know a lot of government pensions that pay out enough for the recipients to even stay afloat, they rely heavily on there being additional private pension funds (401k/BVG), which by themselves presuppose that the economy grows, it doesn't work otherwise. I guess you are right that we have different assumptions on what capitalism even is, you link it a lot to free trade but not to the fact that it is about investment and accumulation (letting the money 'work for itself'), which is kinda the core principle of it. Trade and money has existed before capitalism, exchanging commodities and services is not what makes a system capitalist in itself. I am also not necessarily arguing in favor of socialism, I don't know why you keep bringing that up. Economic systems are more complex than a dichotomy of capitalism/socialism. What I want to convey is that there can be no ecological sustainability withing the capitalist growth imperative. It can be your opinion that capitalism works without growth, but I'd like to see how that works and how we get there. If we can get there, I'd argue that such a system is no longer capitalist.