I agree, but not before Q4 2020. If anyone should know campagn setting glut makes bad business sense it's WotC.
And no, I do not agree 6 months is "a long time". I think it's a very short time. A full 12 months is a bit short IMO.
I don't really disagree but I don't think spacing setting books out really increases sales all that much, because of the peculiarities around buying setting books, which is to say their market is essentially two kinds of customer:
1) People who intend to run the setting, and do. They aren't likely to be interested in another setting book for that reason any time in the next 3+ years.
2) People who buy setting books for the mechanical elements or because they like to read them or the like. They're likely to buy them as fast as they come out, and what makes them less likely to buy them is them being conceptually or mechanically boring, which has little/nothing to do with how close together they're released.
I'm not saying "Speed it up!" or anything like that (nor "Slow it down!"), I'm just saying that unlike some kinds of book (particularly adventures), release rate is likely to have a low impact on sales, especially if you're ensuring they have some broadly-desirable content in them (like SCAG or arguably Eberron, but which Ravnica I'd argue is less successful at).
Major difference is that making books with M:tG material for D&D is something that WotC does now, and has had financial success at doing and have suggested that they will do again.
Have they? That's a little concerning. I like a lot of M:tG stuff but most of the settings seem like a pretty poor fit for D&D and Ravnica did little to dispel that feeling for me. I'm surprised it was successful and would hazard that other M:tG books might be less successful (er, unless they involve Dominaria, then I'm sure they'd make bank even just going over long-term M:tG fans buying it for cool lore and stuff).