One doesn't need to go Bruce Willis in order to have a risk diversification argument. Even in terms of mundane human endeavors, the more diverse humanity's economy and extent is, the less risk there is from economic downturns, the more routine disasters, and the like. That's not enough in itself to justify space development, but it is an advantage even in the complete absence of existential threats.
Further, the argument I noted is terrible. I think it comes about fundamentally because the arguer has a deeply flawed and cynical view of humanity rather than any fatigue concerning pro-space development rhetoric.