From what I've heard, at lease renewal time they've asking for significant discounts. If the landlord doesn't agree, they just walk away. They don't publish a store closures list and try to keep it quiet, but a fair number have shut.A lot of the WHSmith High Street stores are stuck in the early 90s and some have closed recently (Blackpool, Bletchley etc), but more are opening or going through a huge refurbishment programme (Brighton Churchil Square springs to mind).
I'd suggest that makes them more likely to survive though. They just get on with what they need to do to make them the most money (even if it involves closing a store with the only post office for miles around).