Coding doesn't need the assets to be complete to progress. You can use a placeholder to mark where an asset should be and simply replace the placeholder with the real asset with zero or minimal code changes, and this is very typical to prevent programming from being bottlenecked by design. The entirety of the code could be built before a single art asset was fully ready, just waiting for the finalized assets to be plugged in. Moreover, this is a tech update, which means for a lot of the code changes being made there simply is little or even no art work to complete, and in either case art is almost 90% complete and code is still below 30%.
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.