You're ignoring the case of a list being densely packed and embedded inside another data structure. If you bloat a data structure with a useless machine word, there are all sorts of ways for that machine word to hurt the resulting program, and you can't just handwave that away.

The Linux kernel is made by people who spend weeks to shave one bit off the size of a data structure. Do you really think they'd respond positively to your saying they don't need to do that?

You really need to stop assuming what kind of computers other people are programming

The larger point here is you don't get to decide whether other people's use cases are legitimate. Either Rust gives you complete control of the machine or it does not. It does not, not in safe mode, but Rust people keep doing this annoying motte and bailey thing about it.