With the knowledge that BW solo stuff has a limited but still viable audience/pool of potential customers, I think a boxed set (whether physical and digital or just digital) of outtakes would make far more sense than making us all buy the albums again.
It is possible that Playback: The Brian Wilson Anthology was meant to test the waters for a more substantial Brian solo archival exhumation. But I gather it didn't sell well at all.
Anecdotally, I saw vinyl copies being steeply discounted soon after release here.
I don't think they ever expected "Playback" to be a huge seller. He hasn't had any "hits" as such as a solo artist. I think it was just a bit of a prestige thing to remark upon his solo career.
I would expect a "Bedroom Tapes" type marketing job on a solo rarities set to outperform "Playback" with proper hype/marketing. I'm not sure how many even hardcore Brian fans shelled out for "Playback" just for two new songs.