I think in principle the idea of separate smoking premises sounds reasonable. Personally, I'd rather have the smokers shut away in premises of their own, than have them standing in a smoky huddle around the doorway or monopolising the outside seats.
If we were starting from a level playing field, it might work.
But we're not. We're starting from a position where pubs see smoking as the status quo, where they have financial incentives to sell fags, where landlords are fearful that if they introduce a no-smoking rule by choice, they will lose some customers who they know and speak to, with the uncertain prospect of other, as yet unknown people replacing them.
In that situation, I can see why most of them will opt for what they see as the safe option of allowing smoking.
After a couple of years of being smoke-free, I'd be happy to see an option introduced of becoming a smoking premises. That would probably involve having to install effective ventilation, taking steps to protect the staff (yes, even the ones who say they don't mind, same as requiring safety equipment in any other of the settings where we have had to require it), and probably some licensing or planning regulation to preserve some kind of balance. That might involve setting a maximum ratio of smoking to non-smoking premises, and probably also some kind of taxation to remove any financial incentive to become a smoking area.
Something like that might have the effect of creating more choice, without just lapsing into the default position we've had up till now. It might even help reinforce the idea that smoking really is seriously intrusive and unpleasant to a lot of people, and that smokers should be made to practice their habit in a way which recognises this and pays more than lip service to it.