I’m good with this, personally.
I think the proposed requirement from the SRU team to manually test all 40+ extensions in the Ubuntu archive (so long as they are still in there) when SRUing new upstream releases is completely unsustainable, and would result in practice in GNOME Shell updates, containing important bug fixes, not being SRUed at all. This seems like the only feasible way out to me.
On balance I think deferring to the method that GNOME upstream provides, extensions.gnome.org, offers an acceptable experience given this new requirement. In the very slim chance that there is a break in a stable release, the upstream extension author is able to directly ship a fix. This seems very unlikely to happen in practice in Ubuntu until after the problem surfaces in -updates. The obvious downside is that the functional stability provided by the distro is not guaranteed any more.
Interested in hearing the opinions of other members of the desktop & release teams, though.