Mike Linksvayer wrote on 9 August:
there are cases where unambiguously MPL code could be included in a copyleft-next project, if only copyleft-next allowed for it.
Honestly, I'm not convinced that a truly strong copyleft can allow unilaterally inclusion of other copylefted code -- even weak -- without bilateral compatibility clauses. We have to consider that such provisions while seemly safe on the surface may actually be manipulated to undermine the stronger copyleft by enterprising defense attorneys.
Richard Fontana wrote on 2 September:
This proposal might make the most sense for licenses that are generally considered non-copyleft but GPL-incompatible (the proposed language seems modeled on things like the [I believe FSF-recommended] OpenSSL GPL linking exception). For copyleft licenses, it raises the same problem on a more general level that we dealt with when thinking about a clause that would achieve EPL compatibility.
Exactly. I think anyone who wants this kind of compatibility has to prove that it can't be used by the nefarious to circumenvent copyleft.
Note that the ways copyleft is subverted are subtle and complicated. Most people don't see that because they rarely get the rather horrible experience of regularly dealing with people who are trying. You may have to take my word for it that this happens, but if folks want to raise specific questions, I'm happy to talk about it in detail. There's enough of it and it's common enough that it's difficult to generalize.