From what I've read, the sole reason for switching off the PSTN is cost. The kit isn't really obsolete, nor is it non-maintainable, it's fundamentally a question of Open reach getting less and less revenue from the PSTN (because the customer base for copper-wire based systems is dwindling) and the PSTN closure giving Openreach the potential to both cut operating costs by a hefty amount and gain a bit of revenue from the sale of many, now unused, exchanges.
Our nearest exchange is a good example. It's a medium sized bungalow shaped building sat on a fairly good, centre of village location, with houses either side. As a building plot it's probably worth the thick end of £200k. Multiply that by however many similar rural exchanges there are (probably more then a hundred at a guess) and their sale will make a tidy sum for Openreach (or ultimately BT Group, as it seems that BT "rob" funds from Openreach at any and every chance).
I've no doubt at all that the PSTN could be kept going for several more years, but sooner or later is will have to go. OR want this to be sooner (as it's costing them money to keep it going) and some customers want it to be later (because they will lose the means to make emergency calls for a few years). There needs to be a balance between what's in the best interests of Openreach and what's in the best interests of customers.
From all I've read, Openreach seem to have Ofcom, and the government, over a barrel, so it's their needs that seem to be the highest priority, not those of domestic customers. We've been here before, when Openreach decided that letting the Chinese government have access to our comms was OK if they got Chinese kit at a low price. That debacle doesn't seem to have had any impact on the way Openreach behave, as basically they got away with it, in cost terms, despite the hot air from Cameron at the time.
