There have been a number of occasions in the past where a single pool has controlled >51% of the hashrate, but they have not used this to attack bitcoin.
Deepbit in 2011 -
https://bitcointalk.org/index.php?topic=26656BTCGuild in 2013 -
https://bitcointalk.org/index.php?topic=152296.0GHash.IO in 2014 -
https://bitcointalk.org/index.php?topic=645056.0The current level of Bitcoin hash rate is around 437.01M TH/s and the 51% is 222.9M TH/s. The best Bitcoin miner which is Antminer S19 pro only has 110 TH/s and it cost 3000$ for one. Imagine, you have to get 2M Antminer S19 pro in order to make 222.9M TH/s, which is around $6B and that's too expensive. I think there's no bad organization want to take risks on that.
That wouldn't be a 51% attack. If someone was to add new hashrate to the existing hashrate like that, then they would need to add more than all the already existing hashrate.
In your example, if there is 437 TH/s and a malicious party comes along with 223 TH/s, then now there is 660 TH/s altogether. Of that 660 TH/s, the malicious party owns 223 TH/s, which is only 33.8% - not enough to perform a 51% attack. The malicious party would instead need 438+ TH/s, at an even greater cost.
Your math is accurate only if you are taking hashrate which already exists (such as a mining pool) and turning it from honest to malicious, not if you are adding new hashrate.