This is only true in an idealized sense. The balance of pest pressure and host defenses exists on average, but there are plenty of perfectly natural instances where coevolution just doesn’t happen fast enough, or didn’t happen at all.
I mentioned the Florida nutmeg yew earlier, Torreya taxifolia. It’s on the verge of extinction, with the entire natural population in process of being killed off by a native fungus. That species, without human intervention, will be extinct in a generation or two because of a, I’ll repeat, a native fungus. Torreya trees live for hundreds to thousands of years, how quickly do you think they evolve compared to canker and root rot?
White tail deer populations are crashing in many areas of the United States because of a very recent new disease called Chronic Wasting. It’s a prion, there is no immune response to it, nor can deer evolve an immune response because the immune system can’t fight prions. Absence strict quarantine, or some other kind of intervention, white tail deer may simply all die out eventually. No fault of mankind, we didn’t even introduce the disease from elsewhere, it “evolved” on its own and is spreading with and without human help.
Devil facial tumor disease is a completely natural contagious cancer with a near 100% fatality rate that has utterly devastated the Tasmanian devil population, there is no resistance and none has evolved because the fatality rate is too high. There is no cue, and the disease is on track to wipe out the world’s largest carnivorous marsupial. The only successful conservation method has been to keep devils in zoos outside their native range, and then reintroduce them to areas where the cancer has already killed off all the local devils. Without this very direct human intervention, the Tasmanian devil would be on the fast track to a perfectly natural extinction.
Wollemi pine and related species used to be present on every continent, now they’re native only to Australia. Why? Because Australia is the only continent where phytophthera never naturally spread to. We don’t know what wiped out the Wollemi pines around the world millions of years ago, but it’s reasonably likely that it was phytophthera, a disease which was simply too devastating for the host to evolve resistance to in time. We do know the only surviving Wollemi pines aren’t evolving resistance fast enough.
99.9999% of all species are extinct. If nature was actually all about balance, she wouldn’t have brutality murdered virtually every child she ever bore.
We only think we see balance in nature because of survivorship bias.