Why is it now wrong to oppose Corbyn if it once was ok to oppose Mulcair? Was it wrong to oppose Blair?Principled opposition is one thing. Corbyn's opponents have taken it even further and tried to undermine him from the start without ever having accepted the results in the first place or given him a chance. It is especially ironic that his opponents claim that "he can't win an election" when he was already tested in an election, and ended up turning what should have been a super-majority for the Tories into a hung parliament.
Even if you have a bad leader, sometimes the best thing to do is put on a smile, hold your fire, and wait until after the election in order to deal with any leadership issues that you have.
I agree that within two years or so before an election party members should support their leader unless that leader is very unpopular. I think Corbyn is currently not popular but his current numbers are not bad enough to warrant calls for his removal especially since an election seems imminent. However, I do think many people within Labour want Corbyn replaced simply because Labour’s current popularity numbers are weak. I think it’s fair to say that considering the disaster the Conservatives have been in government over the last four years, Labour should be far ahead in the polls.