There is a huge amount of naivety about the open source movement, not necessary by the direct contributors, but by others who hold it up as some sort of panacea – believing and as soon as you make something open source flocks of developers suddenly appear out of nowhere and have an infinite amount of time to work on it. Certain open source projects targeted at a large markets where there are over priced and entrenched commercial rivals such as for a clone of UNIX and web servers are amazingly successful, but for there are tens of thousands of smaller projects which wilt and die through lack of interest or lack of co-operation and leadership. The notion that RISC OS will be transformed by open source is about as realistic as the fairly tail "The Shoemaker and the Elves" – leave the RISC OS source code out over night, and you won’t find the wish list magically implemented in the morning.
Equally there is a misunderstanding of the way the closed source software industry works. Its not all wicked commercial enterprises tightly clasping their closed sources whispering "my precious" like Gollum from The Lord of the Rings. There is a lot of co-operation between suppliers of various parts of the OS and application stack, and they are willing to contribute code to each other in order to implement features that give them a commercial advantage over their rivals. They will not be always willing to make such contributions to an open source OS (or allow one to become open source) as this can give away key information to those rivals. Contracts may even specify API details aren't provided to other users of the OS.
Taking something that's the product of decades of closed source development and contributions from 3rd parties and making it open source, is not a light undertaking, and a potential legal mine field. Sun had to spend thousands of man years of development resources re-writing parts of Solaris before it was in a state where it could be released under an open licence. RISC OS would also require a significant effort to make the entire OS open source, which would be another diversion of our limited resources away from where its needed, which is application development.
Realistically if RISC OS 5 was open sourced, there would only be interest in small parts of it, so it would be better just to make these parts available. I suspect most interest would be in copying the existing Select features such as icon cut & paste in to the window manager, and making the Unicode font manager available for RISC OS 4.X. Things that should have been done by co-operation between Castle & RISC OS Ltd years ago, but show no signs of ever happening. That would be both achievable and a real benefit to all RISC OS users. As for all the grandiose plans of major restructuring of the OS, a lack of resources and focus is likely to make these peter out with in a year or so. Although undoubtedly there will be someone plugging away to make it a "perfect OS" years after the last user has departed through lack of any application development.