A "progressive alliance" is nonsense, and always will be. It assumes that non-Tory voters (except probably UKIP) are united in believing that the Tories are worse than all the alternatives, when this is simply untrue, and it's based on faulty maths in the first place. There are simply not many seats where unifying the left-of-Tory vote would actually win seats. Moreover, there are plenty of seats where the potential challengers are all left-of-Tory; how do you expect the parties to act in these seats? Worse, it reduces the complex variety of opinion to "No Tories!". It can benefit minor parties, perhaps, but it's a terrible idea for any party that wants to form a government in their own right (i.e. Labour) and without Labour it's pretty meaningless.