The definition of an idiot is someone who does the same thing again and again but expects a different outcome. When will the centre-left learn and stop being idiots?
The short answer is when they learn to love and embrace markets.
A slightly longer version would be when they learn to open their eyes to the world around them and observe what actually works. I do have to make one assumption here, perhaps a leap of logic. I assume that what the centre left want is a social democracy of the Nordic sort: given that they continually say that they'd like a Nordic social democracy I don't think this is a particularly huge assumption or leap.
For what said centre left continually seems to miss is that those Nordics are, underneath those high tax rates and high levels of redistribution, largely classically liberal economies. Indeed, one paper claims that Denmark is, underneath those taxes, the most neo-liberal of all the world's economies.
The UK's centre left just doesn't seem capable of understanding what it is that makes what they claim to want work: imagine the horror there would be if I suggested that Group 4S took over the majority of fire and ambulance services in the UK? Yet that is what Denmark does (really: it's actually Group 4S that runs them). We can hear the screams already as Gove tries to bring the Swedish school system with its funding following the pupil, essentially a market, to the UK. Can you imagine the piteous wails if someone suggested importing the Finnish schools system (often ranked as the world number 1)? With its division at 15 into academic sheep and vocational goats?
Compare and contrast the the Swedish health care system with the NHS: taxes are raised in county and spent in county (on average, 400,000 people, it's as if a PCT raised and spent its own money), there are copayments to see the doctor...no, we couldn't imagine the British centre left allowing such a system to exist, could we? Nor the localism of Denmark: the national income tax rate is 3.76%: the top national one 15%. The vast bulk of the money is raised by the communes which can be as small as 10,000 people. You and I would think that money so raised will be better spent when any and every taxpayer knows exactly who is spending it and where they have a snifter on a Friday night.
Or compare the Austin/British Leyland/ Rover story with Sweden and Saab. When GM couldn't find a buyer for it the Swedish govt shrugged their shoulders and said "well, if no one else wants it, why should we?"....a lesson that the American centre left might have noted when they came to deal with GM itself.
Until the centre left stops trying to do the same thing again and again, this insane insistence on the ickyness of markets and the joys of centralisation, then they'll continue to be idiots.
You need to have a classically liberal economy underneath: with all the vim and vigour that implies, meaning that you can get useful levels of economic growth. You might then want to have substantial redistribution on top (I don't, a safety net is just fine) but if you do want the redistribution then you really have got to have the classical liberalism: for there's no other way your going to generate anything to redistribute given the high tax levels you will impose.