I've been reading this wiki for a while now, but only recently discovered this forum, and what I found here shocked me to the point that I had to make an account just to put my own two cents in.
Judging by the posts and comments here right now, it feels like everyone here hates SO, considers it a waste of Oomori's effort, and doesn't see any point in it continuing past SO12.
To start off with, there's absolutely unresolved issues in SO past Volume 12. Has everyone forgotten about the Corrupted Spirit? The one that Revis and Olivas Act both served, "Her", the one that lies below the 60th floor, the one that intervened in the Demispirit fight on the 59th, but Ouranos specifically said had to be dealt with at a later date because they had to deal with Enyo in the present? Well, the later date is now. Oomori was setting this up all the way back then, and now it's time.
Besides that, to me SO fills several functions, both necessary and icing on the cake. It fills in missing pieces from the main series (like why exactly Ais let Wiene go at the end of the Xenos Arc), and gives us perspectives that we wouldn't otherwise see. It's an amazing sidestory IMO. Actually, I don't see it as a sidestory, more like half of the main story that needs to be read in parallel to get the main picture. I actually did that recently, read SO, the main series, and the SO manga simultaneously, and it's great to watch the same events unfold from multiple perspectives. Speaking of the SO manga, I think it's the single best Danmachi content out there; it adds a lot and makes both the main series and SO feel a lot more emotional and alive.