Mainframes can LPAR dynamically. When you want to test if your production system will IPL cleanly, you clone your production environment to an isolated LPAR and IPL it. No impact to production and you get your test.
There were several switch failures in the 1980s / 1990s in which systems which had been upgraded in place without a full restart failed. (IIRC, one burnt down, literally.)
Engineers were uncertain as to whether or not a cold-boot restart was even possible.
When you hotpatch the system for years then you have no idea if the system can boot up or it will fail somewhere in the booting process.
i.e. you can only trust what you regularly test.