But what's the point in buying one if I only need it for a few weeks? I don't want to generate extra trash and those in the office are currently unused.

You don't need to trash it when you're done with it -- donate it to a school or homeless shelter (and you can get a tax deduction!)

Or, even better, just don't buy it at all and bring your work one home with you for the duration of WFH.

How is this even an argument??

Not all companies allow bringing work equipment home.

Exactly what I did.

Take it to your local charity shop

If you don't already have a monitor at home it's because you don't want one. So it doesn't make sense to spend $500 for something you'll use in a temporary WFH situation, then lose money getting rid of it once WFH starts. Much better to just bring the work monitor home for the duration of WFH.

Incidentally I do already have a monitor at home, that costs more than $500. And I'm not using it for WFH, because I find my 15" work laptop sufficient.