This means that a WinSxS folder that is 6GB costs around
I think you just don't understand what WinSxS is, how it works, and what the problem is that it is designed to address, suggest you start reading a bit more.
The reason the old Sxs assemblies need to be kept, is that installed software may require the usage of an old assembly.
Just because an update has superceded a certain library version, does not mean that all applications that still rely on it should be broken.
The SxS assembly backups have a vital role, and they don't actually use as much disk space as you think, due to hard linking --- Windows Explorer gives you an impression that more disk space is consumed by this folder than actually is.
The reason is... various installed files throughout the system will be hardlinked here, causing an appearance that a lot of space is in use here, but in reality --- these hard links are just a second Zero-usage copy of files that are installed elsewhere.
Only a couple gigabytes worth of files that have been updated and no longer have other hard links here, should actually be considered usage of the SxS system.