>But that rewriting the base system, every couple of years
To be fair, they did it once, from v7 to v8. But yeah, a major release does break APIs. I'm concerned with how Drupal Canvas is bolted on, but it looks like it's basically a content editor UI that is separate from the site admin UI, which is a good idea.
I think from v5 to v6, as well. v6 was the last one I worked with (been awhile).
Also, there were a whole plethora of extensions (not just "plugins"). Some of those broke, while others, did not.