"....but it's simply not realistic to expect a modern economy with a health system, pensions system and disability support system to run a surplus for a prolonged period of time without those things becoming dysfunctional, as they are now."
So basically then we must suffer ever increasing levels of public debt in order to finance things that tax revenues cannot meet. Servicing these debts will cost taxpayers £41bn next year. This is over £1,000 for every adult in the country. It is more than is spent on Public Order and Safety, more than is spent on Housing, more than is spent on Transport and only a little less than is spent on defence.
The "certain level of structural deficit" you describe as acceptable, Kromo, is nothing of the sort. It is an enormous burden on the public purse and is only manageable at present because interest rates are unrealistically low. When they return to proper levels the debt will be unsustainable.
Having said that, the "austerity" which the Shadow Chancellor criticises is nothing of the sort. I have seen no "families struggling to feed their children". I have seen lots of families heading off on foreign holidays (during term time) and lots more making for McDonalds, Nandos and KFC whilst out shopping. But I haven't seen any struggling to feed their children, especially when, if they have two children,they get £33 per week to help them do so.