Musically, I think this album is a huge case in point. BW can still turn it on, when he wants to. But getting him to sustain it at that level through an entire album of 12-14 songs may no longer be something he really wants to do.
I actually think Brian has almost always had 'it' since the '60s. He has imo produced songs on a similar level of quality to his '60s peak in every decade since. It's just that he needs a whole lot of circumstances and conditions to be right for 'it' to take full flight. I doubt even he always knows what those conditions are, but he produces the song-writing and vocal goods when those 'stars' are right, as TLOS and the new album show. IMO his songwriting since the late '80s is underrated by some fans.
As to his producing skills, I doubt he has had the consistent energy or interest to truly keep up with all the technicalities of developments, particularly in digital production since the mid-'80s and beyond. I'm not saying he hasn't studied this or doesn't understand it, I just speculate that he's felt a little out of the loop as regards record production since the early '70s, hence the only time he's really produced solo since was on Love You, effectively recording solo (I know Carl gave the Love You sessions a once over after Brian had delivered the tapes) and Adult/Child. Otherwise, he seems pretty much to have 'produced' with collaborators.