Why are you thankful for the MPL instead of the LGPL? Is there any advantage to the MPL other than being easier to incorporate MPL code into proprietary software?
Bottom line is that their licensing with a static linking exception was kind of weird and creating a lot of issues combining zeromq code even with other open source licenses (like Apache 2.0).
Interesting to see how they gathered permission to do this from the developer community. License changes like this are usually hard to realize unless you insist on copyright transfers. But in this case they managed to do it without that. So it was a collective decision. Hard to argue with that.
Yes, GPL compatibility -- in particular GPL v2 isn't compatible with LGPL v3, but it is compatible with MPL.
Several projects, including some I work on, only found out how much a mess (L)GPL v2 vs v3 is once important developers had passed away, meaning it's very hard to get out of the resulting mess.
My project is an IoT node that requires secured and auditable software up and down the chain. We can't allow user replacement. We acknowledge that the spirit of LGPL3 is for a reason and it works for a lot of parties, just not us.
