Rightly or wrongly the majority in this country have no real appetite for left wing governance and I can’t see that changing in the near future, that means left leaning parties have two real options, one is to stick with the principles of their beliefs and continue to be a protest movement with influence but no power or adapt their principles to accommodate the electorate to gain power and make policy that although is a shadow of their principles allows them to at least remove the worst excesses of an ultra capitalist society.
Neither are perfect nor ideal but until those on the left accept one or the other, it becomes a free hit to the Conservatives.