My main thrust was addressed at quality, however
You meant quantity, yeah? And... conceptually, I guess? But the saturation point for any particular published genre is not static -- authors float in and out constantly, just like fanfic authors float in and out of active writing. Especially among the worst authors, heh. It's not a terribly low limit on sheer number of drek, and that's something that's exacerbated by the fact that published stuff tends to be longer per work -- a really terrible fanfic author may never produce a novella's worth of text, but that trashy romance novelist produced at
least a good 100k words and change.
Though yeah, there's a quality difference. I'd just argue that the quality that they primarily differ in isn't all too terribly core to overall work quality -- it's most strongly seen in mechanics, not plot/characterization/world building. With the latter, I'd fairly strongly argue the difference between published and unpublished work is not particularly notable.
Though @ Ree, the WH40k stuff varies
wildly. Some of it's pretty good, just like a fair amount of the Forgotten Realms stuff is solid. It's just some of it... isn't. Think there's some derivative LNs or manga out there that follow similar patterns, heh.