Microsoft can already put out an update to both windows 10 and 11 at the same time. They occasionally do this for bugfixes. That hits basically all Windows users.

The difference in effort between updating one versus two very similar code bases is minor, especially when "Windows 10" and "Windows 11" each refer to multiple versions already.

For drivers you only need one version to support both.

IPv6 has been built into Windows for ages. If you think they can force toggle it or something, that won't work at all and deleting Windows 10 wouldn't make it easier.

I can't think of a situation where consolidating Windows versions makes a big difference. I can think of useful situations based on getting rid of all other OSes and/or Microsoft instantly forcing out updates to everyone, but those are different scenarios and they come with severe downsides.