A mixed economy is a house divided against itself that cannot stand. Sooner or later it must perforce collapse upon its own internal contradictions. Beloved by pragmatists it cannot withstand the full force of a confronting ideology that makes it seem pallid and vacillating by comparison even if the ideology in question is insane as Corbyn socialism clearly is. It holds up an idealistic vision albeit in this case patently dystopic of total state control,fairy tale solutions involving money trees and manna from Marxist heaven run by unions and party aparatchiks hunting down heretics who deviate from the programme and show any recidivist Blairite tendencies.
A mixed economy by defintion is a mixture of two competing ideas of government,socialist and capitalist. This unstable combination and contradiction has to resolve itself either by coming down on the side of complete socialism or complete capitalism.No one looking at the prevailing cultural norms could possibly assert that capitalism is the predominant ideological trend that will triumph when the Conservatives don't even believe in it or dare espouse its premises and fundamental precepts of liberty,small government, limited state interference in the markets (reference May's semi socialist manifesto and speech on entering 10 Downing St).
After decades of compromising and selling out on capitalism,apologising for it,damning it with faint praise offering statist conservative solutions the mixed economy chickens have finally come home to roost and an avowed Marxist came within a few votes of becoming prime minister and may well cross the finishing line next time in the absence of an uncompromising pro capitalist conservative candidate which as far as I can see does not exist at this time.