This is very logical, but it's an engineers view of the world, not a lawyer's. I am not aware of anything in the patent system that requires things to actually work. An invention is an invention in the sense of patent law whether it has any useful effect or not, so long as it's new, 'not obvious' and 'capable of industrial application' (although this requirement is essentially null in practice). I realise that you are talking about validity, not granting, but I'm not aware of any granted patents being challenged on the grounds of 'not working'. Especially not software patents which are generally such obvious ideas that they are obviously implementable, and not having actually implemented them is no problem at all for the inventor, because the invention is the idea itself, not any particular implementation of the idea.
Note that I have taken a lot more notice of European and UK patent law than US, and they do differ in various ways, possibly including this detail of 'needs to actually work', but I doubt it.