Thanks for the detailed reply.

It doesn’t suggest that speculative vacancies are a driver of the housing crisis in Australia.

I was quoting that to explain that speculative vacancy can be a valid, highly profitable strategy for investors. That quote alone isn't evidence, yes.

An investor reluctant to “shell out” for upkeep and maintenance is exactly the type of investor that needs the regular rent income to help cover the other costs of ownership like rates, body corporate, interest, et cetera.

I don't understand how this can be assumed. Investors wanting to reduce cost and risk doesn't imply they need regular income. Rich investment funds would have the same incentive to do that, right?

They do however demonstrate that vacancy is not the cause of the current housing crisis.

Yes, it's not going to be the cause. There isn't going to be a single cause, like you said it isn't even the only cause of mass vacancy. In fact, given it's used as a long-term investment strategy, I suspect that this is a long-term factor that enabled or accelerated the crisis, rather than being an immediate catalyst that suddenly happened a few years ago.