Valar morghulis

I don't care about your reason for loathing The Tree of Life, Charles. Just stop talking about it. It's been a while since you last posted a photo of an attractive, scantily-clad woman, you know...

In fairness, exterminating the multicellular ecosystem would actually provoke fairly drastic changes in many microclimates.
Without bioturbation by worms, for example, pretty much all sediments would be less aerated and more compacted. The removal of oxygen-producing plants and oxygen-consuming animals and fungi would muck up a lot of redox chemistry on which many prokaryotes depend. Many microbes depend on larger organisms for transport or sustenance as well. We need them more than they need us, but it's not wholly one-sided.