Disabling cross-wiki uploads instead of working on new tools to help curate and cleanup those files is a huge step backwards for the good faith newbies who want to contribute pictures to articles.
I am aware of the discussions regarding the lack of ownership - wiki admins do not see the file and Commons admins are overworked, but the solution is to develop new tools to curate these uploads rather than just disabling them.
Very likely, temporarily would be for years in this case.
The whole crosswiki thing needed improvement before it could have been launched at all without causing a massive train wreck. For starters, users should be able to enter "I got this image from www.stockphotocrap.com/photo/253" instead of "own work".
Enabling a feature that causes this much of a mess is not a solution for anything.
Please try to see this from a casual user's PoV: with this feature they doesn't need to worry about where is the picture, how to upload it, how to come back from commons to the article etc. They have a seamless workflow, hopefully in their native language, that makes the picture instantly usable in their article. If we disable it, they'll have to go through another help page explaining how to upload pictures etc. which would decrease the chance of a successful edit even more. This is the problem for which cross-wiki uploads are the solution.
I'm not denying this creates problems for wiki maintainers, and I encourage all of you to log bugs and feature requests against it and push for them to be prioritized, but disabling the whole thing is not the right step forward for our projects.
Disabling the whole thing is, sadly, the only way to get these issues prioritized. The problems are very well known, and developers/WMF/whoever is responsible simply doesn't seem to care. The very least is exactly what I proposed above: allow the user to say "not own work, got it from (insert URL)". It would still leave Commons with the trouble of figuring out the right license, but at least we wouldn't get as many fraudulent "own work" claims. This would be the very bare minimum. Forcing users to claim own work is just.. nonsense. We (Commons) get everything mixed without any way of telling what's what. Fair use (which we don't want), PD-textlogos (which we do want), public domain stuff (which we do want, but please don't claim you are the photographer of a photo from 1900 mkay?), copyright violations and genuine own work. But because everything is claimed as own work, we may be more likely to delete genuine own work if, for example, it is missing EXIF data. By forcefully marking everything as "own work", the claim "own work" becomes meaningless.
Removing task assignee due to inactivity, as this open task has been assigned for more than two years (see emails sent to assignee on May26 and Jun17, and T270544). Please assign this task to yourself again if you still realistically [plan to] work on this task - it would be very welcome!
(See https://www.mediawiki.org/wiki/Bug_management/Assignee_cleanup for tips how to best manage your individual work in Phabricator.)