Who said it didn't cause disaster? There's also the matter of the time course, long-good, short-bad. Which leads to the fact that those that lived at other times evolved in those conditions. The problem with ocean acidification due to man's agency is the possibility that it will happen faster than sea life can adapt. In the past when you get slow change, you get evolution, rapid change brings mass extinctions. Check out those higher and lower times, when they happened quickly, life nearly died out.
That's the way the world works. There have been abrupt ocean acidification trends in the past, in fact the biggest global extinction event of all time was likely one of those.
But every major extinction event has been followed by an even more extreme repopulation of even more diversity of species.
That's the way the world works.
We are neither breaking new ground in creating new negative ocean PH conditions or in creating new warming conditions, or creating new CO2 conditions.
The majority of the Earth's history since life evolved included warmer climate, higher atmospheric CO2 and probably higher ocean PH.
So what's the problem? You think the Earth wants to stay the same forever? That it is supposed to stay the same forever?
What if you are dead wrong and global warming, higher CO2 and increased ocean acidity turn out to be a bonanza for most species? It certainly will be a bonanza for most plants.