> it's clear that it boils down to an unwillingness to continue the conversation, which is inherently somewhat an indication of weakness

Maybe, maybe not. Perhaps Russell had already responded to the fascist position elsewhere, either generally or to Mosley specifically? Perhaps it didn't make sense to dialogue with him at that particular time?

> Just like with communism, where "true communism has never been tried", so too nobody's ever really tried "true" fascism, or democracy for that matter.

I reject this claim, but even if I were to concede for the purposes of argument, they don't need to be tried to be rejected, because what makes them repellent in the first place aren't the supposed ways in which regimes and people have failed to "try them", but the very positions themselves. Both are rooted in a false anthropology and a false humanism that reduces individual persons to means, which further entails a false ethics of utilitarianism.

Absolutely, the technique of "you won't debate me so I must be right" has somehow risen from the playground to mainstream politics, but it's arrant nonsense. Not every idea is worthy of rational and moral consideration, and sometimes it is not weakness to reject even a proposition, simply humanity and a recognition of the underlying motive, which is not always to seek enlightenment, but sometimes to undermine the very idea of enlightenment.