In that case you are only looking at one single update and not the game as whole.
DC himself is does basically nothing but drawing and he starts working on the next update long before the current one is finished.
As such the time it takes to complete the game solely depends on the time he takes to create the art.
Even if they'd do all the things people demand, use placeholders, hire way more people, etc. that fact doesn't change. If DC needs - let's say - another ~4 years to complete the art for the game, no amount of coders, posers, etc. will change that.
It'll always take these ~4 years to finish the game and that's very likely the reason why they don't bother with all of that stuff. It may make people feel a little better, but ultimately it's objectively just a waste of effort and money...
Edit: The only point of time where it really makes sense to get additional help is at the very end. But even in that case DC probably wants to have some time to prepare his next project, so there's very likely no real need, either.