I'm not sure about it: I like when noodles slowly get less hot, and comfortably drinking the rest of the liquid in the end. And in a thermo cup, if it's too hot, it'll stay too hot
Nah. Just having the lid off to eat the noodles is enough to let out most of the heat. My stanley can keep water near boiling for a good 10-12 hours with the cap on, but even slightly open that time drops down to an hour or two at most. It's really wild how significant the difference is.
You can't scare a welder with popsci nonsense. Why would I ever worry about a steel capped lead seal that's as far as it's possible to be from the liquid in my cup? Might as well worry about breathing nitrogen from the atmosphere.