It is a reactionary response to Capitalism’s decay

Interesting theory, need to think about that. Though I don’t think Fascism and Capitalism are necessarily exclusive. (As example China, which is I’d say fascist and against the usual theory of “communism” quite state-capitalistic)

If you understand that the rate of profit trends towards zero, why would you think Capitalism is stable?

Because the trend is clearly not to zero currently, there’s a lot of rich people (and the number is growing), profit is still growing, it’s just that the gap between poor and rich is also growing. But Capitalism doesn’t care about a lot of poor people. How that will be long-term is another story (as said ecosystems are the limiting factor).