Yeah windows "cumulative update" upgrades for the past couple of years basically duplicate the whole system directory and apply the update to that leaving the existing one to roll back to if anything fails

Windows updates (and Windows Installer) are transactional. If the update or installation fails, it knows exactly how to revert back to the previous state.

Windows Installer supports this across multiple packages too - for example, a game might need some version of DirectX libraries which needs some version of the Visual C++ runtime (probably showing my age because I doubt games come bundled with DirectX any more). If one of the packages fails to install, it can handle rolling everything back. Linux can sometimes leave your system in a broken state when this happens, requiring you to manually resolve the issue - for example, on a Debian-based system if the postinst script for a package fails.

But also I thought Linux distros normally keep the old Kernel around after an update so stuff like this doesn’t cause a boot failure?

Arch has no concept of "previous package", so it doesn't do this.

You could install linux-lts (or one of the other alternative kernels) side by side with the linux package, so you always have a bootable fallback, but like most things on Arch it's not enforced.