The content is not the only valuable thing. The community is. And communities are hard to move.

If, tomorrow, some angry internet god partitioned things so that the disappointed moderators left and set up the old database at a new URL, and at the same time wiped the original sites, the original sites would still likely be most popular this time next year.

The law lets you do that, but Google doesn't look kindly on sites that are 99% copies of others, mostly because spammers do that a lot.

Actually, anyone can download their entire [anonymized] database (or select only some sites) from Archive.org via torrent or directly:

https://archive.org/details/stackexchange

I thought content there was cc-by, how then is it anonymised?

Good question, I have wondered about this myself.

Also because after I deleted my account all of my 2000+ answers are now attributed some user012345, not my username.

Seem they do whatever they want with the license. They just changed it from v3 to v4 of the cc by-sa with attribution required and without mine and many other's permission...