Why does nobody think a boxset is likely?
Depends what is meant by "box set." I could be wrong, but I'm not sensing a big box set like the one they did for BCSL with the LP's, mouse pad, etc. Something like that is expensive to package, which means it is expensive for the fans, which means it isn't a big seller unless either (1) it is for a big, momentous occasion, and/or (2) it has a lot of really cool content that a LOT of fans want to buy. Since they just did a big box for their 10th album, I don't see one this time around.
But there will almost for sure be a SE with a bonus DVD. JP said they planned on releasing a DVD with the documentary. I know the plans changed drastically during the process, so maybe that was an earlier plan that has changed as well. But since they have it professionally recorded and edited, it would be a shame if they didn't make it available for a special edition, and I'm sure there is a lot they could include as "bonus" footage of the audition process, the making of the album, etc. That being said, it wouldn't surprise me at all if they did a 3-disk "box set" like the smaller SE of BCSL or like The Whirlwind, where they release a package with the standard CD, a DVD, and a bonus CD of covers, bonus songs, demos, and/or instrumental tracks. A SE would be a good way to make a statement to the fans that just because Mike is gone doesn't mean they won't continue to go above and beyond in giving extras to the fans. And, again, given that they already have this documentary footage, which means they've already got pre-made video content, and given that they have scheduled a release date that is slightly farther out from the time the album was in the can than the normal release cycle, it just seems logical to me that there will be at least a CD/DVD SE, if not a more extended SE.