By looking at the last thread and this thread I think the following people support the proposal:
Johan Tibell
Michael Snoyman
Christian Maeder
Henning Thielemann
And the following people against:
Ganesh Sittampalam
Ivan Lazar Miljenovic
Erik Hesselink
There were also lots of discussion by other people and, as per the rule on mailing lists discussions, discussions that weared off in a bunch of directions*.
Some comments to help further discussions:
You can still write "orphan" instances by using a newtype, if the instance is only used internally. I did this in e.g. ekg which needed some aeson instances. It added a couple if line of code to the library as a whole.
You can still write orphan instances, you just need to have tighter version bounds.
There were some comments along the lines of "I prefer major version bumps to breakages." This doesn't introduce breakages, as long as people follow the updated PVP.
P.S. Other core library maintainers and I have already avoided bumping the major versions in several libraries, including containers and hashable, in the past, as I knew that would require more or less every package author to release a new version of their packages. In other words, we don't quite follow the PVP today and I don't think we should (i.e. we should change the PVP to match current practice.)
* This is probably what I eventually abandoned the discussion and it's something that has annoyed me about libraries@ discussions for quite some time. We, as a community, need to get better at concentrate on the technical discussion at hand. We cannot redesign Haskell in every libraries proposal thread, as fun as that might be. The alternative would be to have less community input on these decisions -- which I think would be a shame -- as is common in other language communities.