>There's zero reason to change your URLs every time you change CMS, and there's even less of a reason not to redirect the old URL format to the new one.
Are you willing to pay them for that work?
No?
There's your reason.
Are you willing to pay them for that work?
No?
There's your reason.
I think a lot of folks originally thought of websites like reference books. And some should still be thought of that way (Wikipedia, open source documentation, etc). I used to try very hard to keep all URLs functional during site redesigns. We accumulated thousands and thousands of redirects… most of which were never used.
I’ve since come to consider that a lot of sites are more like magazines: useful for a limited time span but not something that needs to live on your shelf forever.
If we as end-users want URLs to not rot away, we need to put value on working URLs that convinces webmasters to put in the effort to maintain working URLs.