I wrote how DC would not have to waste his time on doing sketches for placeholders by hiring people for that so that he can work on the additional stuff.
I don't think I was clear enough. Right now he isn't working on any additional stuff, so there's no time to gain from where we are now.
And I'd disagree about the placeholders specifically. Placeholders will not increase the development speed of the game from start to finish, simply because it's not even about the time it takes to draw them (as you say, that's quick), but about the time it would take DC to explain to somebody else what he wants. In the time he has done that, he may have drawn the sketch on his own. If you provide a finished image, you may have to explain a few things once, if you start by talking about the scenario without any kind of sketch, etc., the process simply gets more time consuming and you may even have to talk about the same issue twice. In the worst case that'll cost additional time and as such will make the time it takes to finish the game even longer.
You are still only looking at one single update and kinda ignore that it doesn't matter for the overall speed, if coders have nothing or - temporarily - to much to do. DC is working continuously for years now and there's just no time to gain. If he had done all the things you say in the past (since he started to hire peoeple, of course), the game would still be where it is today. Sure, maybe a littel better polished code wise but there wouldn't be more art around.
We simply are talking about a two different issues. It's fair to say that hiring another coder would very likely improve the developmentprocess as a whole and that it would probably make the game a bit better (for now). But none of that will make the time it takes to finish the game shorter and as such art is indeed the limiting factor.
As such, the only thing that would speed up the process is to hire another artist and once that is done, one would have to see how many coders oder posers are required by the amount of content two artits could produce.