Its pretty obvious to anyone who has looked in to the RO5 HAL model that it is the way forward and superior to the mish mash of procressor and device specific code in the Select, RO4 and earlier kernels. The mention by ROL that certain Select features can't just be softloaded on top of RO5 because they are dependant on things in the Select kernel, reinforces this point, as those things should not be in the kernel to start with, but suitably abstracted.
If we are to prevent any further wasting of time and effort on duplicated OS development, and gain a true single source, one size fits all OS, ROL must licence the underlying RO5 kernel and HAL from Castle and add their value on top of this with Select features. There is then a clear well defined boundary between device specific and general OS code, both technically and commercially. The resultant OS can then be deployed on the Iyonix, A9, Risc PC and other legacy machines, bringing its advantages of hardware indepenence and enhanced APIs to all RISC OS users.
Will it happen? - No.
Paul Middleton is on record as describing all the other players in the market and Castle in particular as "Bastards Bastards Bastards", and their cheif programmer (if he's still working for them) views the minor internal divergence of Select and RO5 as schism of religous beleifs, and about as likely cooperate on integration in to a single source as as sunni and shia muslims are to get back together in Iraq.
Castle also view working with erratic management style and financially untransparent ROL with horror. They've never really seen the Select features as something which sells new machines, and while they might welcome the ultimate goal of a single source and letting ROL play with the UI bells and whistles while they concentrate on hardware and embedded projects, getting there is going to be nothing but pain. They could be faced with the intermediate step of a Select over RO5 bodge generating a nightmare of support issues, which users wont be able to distinquish between problems with hardware for Castle to deal with and OS bugs for ROL to fix. A two tier local governement style buck passing ring is bound to deveolop damaging the reputations of all concerned.
My personal view, which I'd very dearly like to be proved wrong on, is that there will only be one source again when there only one company, i.e. a merger or more likely one or the other going to the wall. My bet would be the one that has taken its customers money for 19 months and failed to deliver any kind of product.