The biggest mining pools have known identities[1]. If some unknown group started mining and the hashrate jumped 50%, it would draw lots of attention. There's no way it could be done secretly. A more likely attack would be a government bribing or coercing a few of the biggest mining pools to manipulate their blocks.

[1]: https://btc.com/stats/pool

How unlikely is it for that not to have already happened?

Bitmain are using the latest process nodes for their ASICs.

Not GP, but this isn't something you'd need to calculate. 130nm transistors are so much larger, with so much higher power consumption to modern 7nm transistors, that it'd be like comparing the fuel consumption of a motorbike to a school bus.

Bitcoin mining reduces to a process of energy conversion. Electricity goes in and money comes out. The latest and greatest mining gear is much more profitable than older gear. As more people buy the new stuff, they are able to run more calculations with the same amount of energy. This pushes up the cost of mining for everyone, since more hashes per second are happening for the same energy input.

Those on less efficient gear eventually get pushed out of the market because they can’t make a profit.

A 5nm node is many many times more power efficient than 130nm. There’s no contest. An ASIC with that kind of power draw would not be a profitable mining processor.