I don't understand the "he's unelectable" argument. Or rather, it seems to me not so much an argument as plain propaganda on the part of the neoliberal establishment - politicians, media, corporations and the shadowy government departments who prop up the status quo. Keep saying "of course, he's unelectable" and, without any argument, you tarnish someone's image in the eyes of the underinformed public: "the experts seem to think there's something wrong with this guy." Similarly, you can brand left-wing politics as impractical or unrealistic - "of course, that wouldn't work in today's world" - well, maybe it wouldn't work in the face of the overwhelmingly powerful neoliberal establishment, as we saw with Greece's treatment. But "support the powerful because they tend to win" is not a good motto for life. I consider it healthy for a country to have a good political spread. The UK has been sick for a while with only of various shades of right-leaning post-Thatcherites on offer. Corbyn said yesterday that Britain should take a lead in nuclear disarmament. To me it's refreshing to hear that brought back into the mainstream debate after a generation of Blairy cowards didn't dare touch it. A Corbyn-led Labour party might be jeered at but it would once again give a voice in Parliament to many who would put the goal of a happier society before the continued gains of the already powerful.
Those proposing he's unelectable generally believe that a leftist government is a thing of the past in Britain – the last leftist government ended in 1979 following the Winter of Discontent, a series of widespread strikes in many industries reacting to poor economic governance. Margaret Thatchers' Conservative government was elected directly as a response to the poor performance of the previous Labour government. Labour remained in opposition for 18 years, until Tony Blair won the 1997 election with his New Labour campaign – a restructuring of the Labour party to appeal to the centrists, inspired by the Clinton campaign in the US. Many in the UK–including Tony Blair and many of the elected Labour ministers–argue a Labour that doesn't appeal to the centre can't win. I would argue that since Labour lost its large Scottish base to the Scottish National Party (SNP) in the 2015 election, a centrist Labour government can't win. A shift to the left will give the much ignored left-learning portions of the electorate something to vote for again, hopefully increasing youth turnout and winning back some of the Scottish voters who went with the more left-learning SNP; and may well allow Labour to ally with the SNP to form a coalition government. Either way, I don't see Labour winning again in its current incarnation; unless the SNP royally screws up and fails to keep its current voters. Something has to change in Labour.