Although the impact on BBC national radio would have been limited, as Holme Moss and other sites further north would have still been in action, reception can be patch in many areas, including in Manchester itself. I know I always had better coverage on Winter Hill's frequency rather than Holme Moss's, which was affected by the terrain and multipath from the buildings. It also wouldn't surprise me if Winter Hill formed part of the microwave distribution network, so an issue there has the potential to affect services further north.
Very little microwave dist is used these days without back up, and where it's the only delivery method, (for the local DAB muxes for instance) the distibution always arranged in a ring.
I'm not suggesting MyP etc offers an alternative source of reception should Winter Hill fail, just that it will provide alternative reception for some immediately, (by just going to 800+ on the EPG
It wouldn't require a very tall mast to restore TV to Manchester, the top of Winter Hill (the hill not the mast) is visible at street level in parts of the city. Of course locations further away are a problem, as would be access to the hill to arrange an emergency service
It will be interesting (albeit expensive) if WH does fail, to see what the real impact on radio and TV reception actually would be, it certainly wouldn't be a cataclysmic as it would have been 20 years ago,