I posted this as a comment on your video, but I thought that I would throw it here as well:
I don’t disagree with a lot of this, but missing here is the theory that at least one of them has children at this point. They mentioned spending a lot of time cataloging their music because of this in an interview during the TH era. Part of me wonders if BOC, idk, got lives (for lack of a better term) and so they aren’t able to dedicate as much time to the studio, which is why this next one has taken such a long time. They’re perfectionists, work hard, and it’ll take as long as it takes, which obviously will take longer when they have other life priorities. I can only imagine how tedious of a task it would be to do something like clear samples or even finding samples, let alone everything else they do while being a Dad and/or Uncle and such.
Fans sometimes forget that the artists they follow are human beings with human obligations, especially as they do things like start families and especially when they are in art & business together as brothers. There are a lot of interpersonal dynamics that go into Boards of Canada that the fans don't necessarily see because of their public mystique.