Simple answer for most end users will be :  Dollars invested over the last 10 years or so. They have tons of texture sets and closets full of clothing and support items.
Vendors have a proven and stable workflow using proven tools and a solid market.
M4/V4's future? At least the next decade or 2, if not more.

Digital content does not age over time.
M4/V4 will look exactly the same in 30-50 years as they do now.
And they clothes will not turn yellow over time either. LOL.

Hey? When we see how many out here are still on Poser 6, and 7,and 8? 

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!