Don’t know why. In 2018 the tools and assets are there for a bunch of kids working out of their bedroom to produce an AAA result using those resources and if work smart produce a result equal to a top shelf game development company. Unreal 4 for example is an engine that once would cost you a million dollars to license which for the most part is free until you start making some money. All that is required is time and effort to learn from the top down. 
Re-targeting is just the process of renaming animation set using one naming convention over to another and adding this as a feature in UE4 is just a convenience for the have nots. Using MB, expensive I know, one does not really rename, re-target, but rather characterize the animation data to a standard and save the result. This way there no difference what rigging is being used, being Mixamo UE4 BVH or any other source, and it’s a simple matter of drag drop and plot. Working smart it’s kind of like making a web page where one can mark up elements with out knowing what the under lining HTML code is doing or even knowing what any of it means.
What would be nice is if UE4 could save a template that can be saved. For example from Mixamo to UE4 so that the file can be shared.
I figure another 5 years or so re-targeting will go the way of the VHS tape.
Well there is working from scratch and then there is working smart and working smart means letting the software do the job that it was designed to do. Once you have the perfect base shape, the perfect rig, the perfect UV mapping and the perfect morph targets there is no reason one can not use that base to create any character actor you wish with out having to reinvent the same wheel over and over again.Working smart usually means working faster which leaves more time to do things perfect that one would have to surrender due to the lack of time.
For your needs this would be a very good starting point as you could accomplish the top down requirements that you could using Genesis but just from a different direction. Checking the licensing the base assets is under GPL where the output is covered under CC0… Good to go as in what you make you can sell.
Just to confirm yes it’s a hard decision to make, we spent two years deciding on Genesis, as once you make that decision you will probably have to live with it for a very long time.
Morph targets are now GPL rendered and no longer a performance hit.
To make use of the G3 frame work you have to make use of the G3 base mesh. The licensing says though that if you make a derivative using the base mesh then the derivative becomes your property to do as you please with out any additional fees required.
As an exaple if you export the G3 mesh to Zbrush, and use the tools to make a morph target, and change the base G3 shape by a large margin and in the process create 100 unique shapes then as a derivative all 100 belong to you as property. Think of G3 as a fancy box primitive that happens to have arms and legs.
The flip side is one only needs the G3 base rig and naming convention, which can not be copyrighted no more than Ford can copyright the wheel, and even says in the licensing that Daz3d encourages this as to making new products that you can give away for free or sell if you wish.
As a heads up though the reason we went with Genesis is because Daz Studio is in the millions and millions of downloads but has not really been used as a games development tool until 4.15 and the introduction of G3 so it’s complexity has a high learning curve. To try to fill in the gaps I’ve sent a few e-mails off to Daz3d asking for permission to put together a UE4/G3 SDK so the framework can be tested in hand.