Who is to say the failure of social democracy was not because of the historical conditions of that time or flaws in the institutions/political culture/tactics of socialist activists, and that we can't learn from these mistakes? Or perhaps things have changed so much that social democracy can now be used to abandon capitalism? These are at least things worth having a serious discussion about rather than dismissing outright.
We can look at the factors that drove the post-war consensus - a strong, militant w-c, capital's demand for a educated workforce - and see whether those factors are present today. They pretty clearly aren't, capital is now pushing for a larger "unskilled" workforce in order reduce the pay and conditions of workers even further. There are also the changes to the global political situation that IMO make a return to something like the post-war consensus unlikely. I think the post-war consensus arose out of a particular set of circumstances and those circumstances no longer exist. The post-war consensus is gone, and I've not seen any social democrat really address this.
We are all prisoners of our historical circumstances, and right now I cannot find any more productive use of my time politically than backing the Labour Party. (and I'm also involving in Diem25). Even if I took your objections to social democracy on board, and I do take them seriously, it means absolutely nothing practical to me and would just mean I go from supporting Labour and trying to push it leftwards to, well, doing nothing at all really. And you can't just blame me for not understanding it; if your politics are hard to figure out how to get involved in then that is a problem with your politics which you need to figure out a solution to. Left of Labour, the Trots are actually the most accessible but you also seem to have contempt for them too.
(my emphasis). Of course you'd still be doing things if you weren't in the Labour party. You'd still be working, you still interacting with your friends and family, all of that is political. The idea that the alternatives are joining the Labour party (or Trots) or doing nothing is nonsense, it's based on the assumption that the LP, or any other party, is the driver of change. It's not, the only force that is capable of improving society is the action of labour, is working-class insurgency.
Now you might feel that the best way to maximise the power of labour is to join the LP. Personally I think that is the wrong move, there's no shortage of examples of people that have joined the LP in order to use it only to end up being used by the party, nor of the examples of betrayal and selling out. However, while joining the LP is not a course I favour I do understand why people are doing it and there are people I consider comrades in the LP. But it's vital to recognise that the LP (or any party) cannot be anything but a tool, a means to an end. The post-war consensus wasn't brought about strong social democratic parties*, strong social democratic parties were brought about by the post-war consensus. Likewise any fight back against capital in the modern world will not originate from Labour but from labour.
As I said, personally I think joining the LP is a wrong move, and I'm perfectly willing to have that debate if you want, but what I'm criticising of SaskiaJaynes (and others) posts is not people joining the LP but
the replacement of the working class with the Labour Party - or anything party/movement for that matter. You mentioned the failures of "parties guided by marxism", by which I presume you mean communist/socialist parties, well I'd argue that those failures were brought about by thinking I've outlined - the supposition of the party, or state, etc, for the working class. For me socialism means the workers control of the MoP, not the states/parties control of the MoP.
*It's worth noting that for much of the post-war consensus across the west centre-right parties were often in government and the centre-left in opposition, here, in Australia, in NZ etc.