Because the quality is also massively improving, the standards are increasing.
Comparing this
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with a low quality stock background, weird remnants of the old background, generic pose to the stuff he's putting out lately, or the stuff I'm seeing him work on dev chat now... it's night and day. The newer stuff requires often a shit load more time in photoshop, in SD, etc. The writing is also being taking more seriously now as opposed to early on. The thing he's been working on the last couple days for example requires a lot of visual design, but I think once people see it those who appreciate visual design will get a real kick out of, it's professional level work vs the early casual stuff but that takes time.
And no, before it devolves into another back and forth he has no interest in sacrificing quality for speed. I'm not saying you have to agree with the why, I am just telling you why.
The story did progress in the last update a lot, openly talking about sharing, Eric getting his first threesome, Sai developing into a cuck, the next photoshoot being brought up. I know very well what you actually want, it has less to do with plot advancing and a lot more to do with one specific fetish. You'll get (some) of what you want eventually, but there is more than one thing going on.
In any case, development is going well the visuals look (in my opinion) better than ever with a visual style closer to what a lot of people liked about the chapter 2-3 window but with some new flair, but I won't pretend it won't take quite awhile to finish.