jess answered that rather eloquently... No, I wouldn't miss Select, as I've not had the pleasure of experiencing it - and even if I did, Castle are encouraging a version of Select for RO5. I can't honestly see that RO4 and RO5 are both going to be developed in tandem in the medium-to-long term - it just doesn't make sense. A version of RISC OS for older hardware can't just "go away" - as the majority of hardware out there is older, however much MD, Castle, et al, would like to encourage us otherwise.
Given the sheer resources that Castle have compared to RISCOS Ltd in terms of development (the point simply being that Castle have some, and RISCOS Ltd don't appear to really have any), I can't see RO4 going anywhere, while RO5 will continue to evolve. The effort required to continue backporting everything that isn't new-hardware-specific will surely be a lot more than stablising the HAL for existing platforms.
A question, though: What would I REALLY gain, besides some aesthetic niceties, by having RO4/Select instead of RO5, and would it justify the subscription? All Select seems to be is a few bug fixes, some prettying (if that's a real word, which I doubt), and a few things conveniently packaged up/now in ROM which are otherwise available (and perhaps the odd one or two which aren't). Apologies if that doesn't fill me with enthusiasm, but what happened to the world where bugfixes don't come with a hefty price tag?