That's always been my biggest gripe with other distros, they'd have a version of an app or tool that was built once years ago and never touched again. I'm sold, I'll be taking Tumbleweed for a test drive tonight.
I run antergos (arch based) at home, having the bleeding edge is nice, but the AUR is really what seals the deal for me. That being said I have been cut by the bleeding edge at least twice, once so badly that it booted up without the eth0 interface. Maybe tumbleweed wouldn't have this problem as often.
http://software.opensuse.org lets you search across all the available repositories and easily install packages on your Tumbleweed installation
Sure, those repositories are not tested, unlike the main Tumbleweed distribution, but then again, AUR isn't exactly expected to be a wholly reliable resource, just a useful one :)
In the case of UI changes, openQA can have it's 'needles' updated in it's webUI. Look at the new screenshot, compare to the old screenshot, tell openQA to like the new stuff, done.
In the case of functionality or workflow changes, yeah, tests need to be adapted, but the openQA test writing language is pretty human friendly, describing the steps that humans actually do, so it's not that hard to alter
And because we test at the point of submission before anything is merged to the distribution, we generally catch the behaviour changes as part of the package submission, then we have developers keen to get their stuff in the distribution happy to help tune up the tests ;)
A lot of Debian-based distributions (including the first Ubuntu releases) are basically Sid snapshots at some point in time. It's been like that for a long time.