They were hugely disruptive and set off a massive extinction event... not to moralize the actions of cyanobacteria (who obviously act without agency), but the only reason we think what they did was fine is that we breathe oxygen!
This is the brilliance of natural selection. If it were any less robust life would probably have already gone extinct. It's "easy" to devastate the ecosystem and wipe out say 90% of biodiversity, but that remaining 10% becomes increasingly difficult to root out, as you're increasingly dealing with the hardened survivors. The massive bust leaves open ample under exploited niches, which then spurs rapid disruptive selection, and then subsequent sympatric speciation. Long after humans are gone something will probably be thriving feasting on our garbage.
Well, sure, there's stuff like anoxic life that lives off dissolved hydrogen in rock and will be mostly unaffected by anything we do. But the vast majority of life on Earth is exploiting the huge energy gradients created on the surface by sunlight and exploited by photosynthesis, and that's what most people are thinking of nowadays when we talk about "life on Earth".