Redshift is a renderer that does a specific job with its own specific approach, it is a hybrid but it is still not the all go and be all solution and has tons of features still missing and other limitations for full blown productions (as is the nature of GPU renderers). This discussion about GPU renderers have been made countless times before and there are very valid reasons as to why GPU rendering is not picking up, at least not in the foreseeable future yet. If this was the case then all the big studios and all the best renderers such as Vray, Arnold, Renderman , Hyperion (Disney) would’ve all gone the GPU way, and some of these renderers have been recently rewritten entirely.
Also GPU is not “Cheaper” Than CPU as purchasing a couple of old xeons with 32 gb - 64 gbram to make up a renderfarm in a small room would easily outdo any modern GPU card price wise plus give you a solid tested pipeline without surprises.
Also putting your entire production pipeline of rendering at the mercy of Nvidiia with its “super cards” is a suicidal situation and you would be asking for trouble.
You are upset waiting 2 days for a render? Welcome to the world of rendering, some of us coming from VFX world would be so happy to just have 2 days worth of renderings! when we wait months and weeks for a one minute scene sometimes. 
If there are any improvements to be made to lightmass, we hope it would adopt a more Vray like brute force/light cache hybrid approach for baking GI vs the old mental ray type final gather approach. That should usually bake nicer GI more accurately perhaps even faster, but then again games have low res lightmaps and this quality over resolution can become questionable at some point as to if it is worth implementing it at all. But I think it will only benefit it. Also by the time if lightmass is updated in the far future we would probably get dynamic GI performant enough to make it production ready. But that’s just wishful thinking.