It feels like you assume OSL is Cycles only ? Sorry if I guess wrong
SPI Imageworks created it for their production needs with their Arnold fork. It is now implemented in Arnold retail, Redshift, Renderman, Octane, Clarisse, 3Delight and Octane. It is big for everyone because it opened a lot of possibilities, it is simple to use and it is quite efficient.
MaterialX is promising but its main feature, the interoperability of shading networks, to me, is specially interesting for big production house where they might need to use multiple rendering engine in one production, or share big chunks of data to another studios. Is this useful for blender/cycles users ? Is it a top priority ?
I don’t have answers to those questions, but I believe the blender foundation is in the best position to judge if the amount of work needed to implement it is worth it or not, and at which priority.
I would personally prefer texture caching, better volume rendering, deep id support and some CPU rendering love hehe, but that’s my own wish list