I'm pretty sure every movie company you named could have rendered the same thing in Cycles just fine. You still can't name a single important thing that can't be done with Cycles.
Speed is good, but you cherry picked some niche thing that's probably faster than Cycles (which you haven't provided evidence for). That's a silly argument.
The burden of proof here is on Cycles. There's 30 years of history showing off Renderman, and everybody in the CG industry knows that it's awesome. Not many people have even heard of Cycles. What movies have used it? Why is it better than the existing rendering software? Is it compatible with the Renderman spec? How easy is it to set up a render farm with it?
This. In theory Cycles can be used to make the same kind of movies Renderman can. Please name a few examples of cases where this was done.
On the flipside, when people say Renderman can be used to make production level stuff, they can point to literally any film with CG made in the past 25 years as evidence.
Let's say you want to render realistic skin using Cycles. You can't, the only BSSRDF profiles that Cycles offer are simple bicubic and gaussian profiles, these wouldn't work for rendering realistic skin. Cycles doesn't even offer a simple dipole. Renderman on the other hand offers a dipole, it offers the state of the Photon Beam Diffusion, it offers Pixar's recent approximate BSSRDF, which is almost as accurate as PBD and is as easy to compute as a gaussian or a bicubic profile. So you'd have to implement your own BSSRDF shader, which is only doable for large studios such as Weta which have their own R&D department.
So let's say you want to render hair. You can do this with Cycles, but their importance sampling code is not state of the art (last time I checked), which means that you can probably expect double the amount of noise in Cycles. You could still use this for rendering, but computing time is quite limited (especially if artists want to compute previews). You'd either have to implement your own hair shader, or you'd have to purchase a ton of extra hardware.
Let's say you want to render a scene with tons of triangles and textures, you probably won't be able to do that in Cycles. They don't use special tricks for quantizing those triangles in the memory. Their code for caching textures (sometimes multiple terabytes for a single scene), is also not as good as Renderman, especially when using the GPU (a GPU isn't very good at constantly streaming terabytes of data into it), which means that your texture date either needs to be limited, or you'd need to throw a lot of rendering machines at it.
Cycles can do most stuff that Renderman (or Arnold) can, but that's not the point. If you'd pay me to work on a renderer for you fulltime for a year, I could produce a feature complete renderer, but it won't be optimized. The code won't be optimized, but probably more importantly the ray intersection code and the importance sampling code won't be optimized, which means that renders will be slow and noisy. Pixar has a whole team working on optimizing their code and whole team of researchers working on improving the importance sampling. Cycles is made by hobbyists who are doing an excellent job (Cycles is an amazing renderer for amateur users), but it's just not in the same ballpark as Renderman or Arnold.
Cycles is an absolutely amazing renderer, but it just lacks a lot of stuff that you'd during production of feature movies. If you want to use Cycles for your own use I can absolutely recommend it, it will have everything you need and it's open source!
Just one nitpick, Arnold supports the same BSSRDF profiles and has been used for realistic skin rendering in movies. The gaussian profile is actually suitable for rendering realistic skin, by using a combination of multiple gaussians you can very accurately match measured human skin profiles.
Yeah, such profiles can certainly be used for skin rendering. I know that SPI used the method you mentioned on a few movies. There will be visible flaws though. The skin and especially the lips will look too waxy when using gaussians or bicubics. Weta Digital used Quantized Diffusion on Promotheus for this reason [1]. Pixar has an upcoming talk on a simple, yet highly accurate BSSRDF profile this SIGGRAPH, so let's hope the folks at Blender will implement that in Cycles.
> it will have everything you need and it's open source!
Not everything for animation, yet.
BTW, for those who don't know, solid angle is the company behind the Arnold Renderer, and the creator of cycles, Brecht Van Lommel left the Blender Foundation not so long ago to work for them. The quantity of people contributing to Cycles is pretty small compared to other areas of Blender.
Speed is good, but you cherry picked some niche thing that's probably faster than Cycles (which you haven't provided evidence for). That's a silly argument.
EDIT:
Thanks for the updated post, much more substance.