I remember back in ~2015, one of the bigger mining farms was rumoured to have >50% of the hashrate, and even then, it wasn't catastropic...
it wasn't a mining farm it was a mining pool called Ghash and it was lots of individual miners (and also their own hashrate) that were connected to that pool, when the article about the risk of 51% attack was published and these individual miners saw the possible
threat drama they simply switched to other pools and their total hashrate dropped.