Full Support For RenderMan

There’s a lot to unpack here.

You seem to assume that Blender limits the performance of external render engines by forcing addons to be written in Python, and that building RenderMan into Blender would somehow solve that. That’s not really how it works.

I'm not an expert, but while Blender’s only available API is in Python, that doesn’t mean everything in an addon has to be. Many render engine bridges use Python just for the UI and scene export, while all performance-critical tasks are handled by native libraries i.e. written in C++. AFAIK, the only actual bottleneck imposed by bpy is when reading Blender’s scene data, everything else is fully in the hands of the bridge developer.
It's the case for Cycles , and so is prman-pixar/RenderManForBlender : the integration into Blender is in Python but Renderman remains compiled in its native code developed by Pixar, with Blender being a mere bridge between your scene and the renderer.

As for "building RenderMan into Blender", which I guess you mean as in being natively present in Blender out of the box, that’s unlikely for multiple reasons: RenderMan is proprietary and not GPL-compatible, the Blender Foundation focuses on open-source tools it can maintain, and Pixar’s existing bridge is already functional and supported.

I use Octane for Blender. What if Blender gave support to other render engines, and required them to give a percentage of their subscription fee? Octane costs 23 euros per month. What would happen if the price increased to 30 euros, and 7 euros went to Blender — but only if Blender provided official support for Octane, or for other engines like Renderman, V-Ray, etc.?

GPL 💀

It can't be done due to GPL license.