Outside of a dictatorship, it's usually considered healthy and normal that people can criticise their own leaders. It's hardly as if Theresa May had the undivided support of her own MPs, was it?Except that the result in 2017 wasn't just deflated by party staffers doing what they could to sabotage the election campaign. The PLP and Labour right had been publicly attacking the leadership at every opportunity for two years to try and be "proved right". So the largest popular vote since Blair's 1997 landslide no doubt did come as a shock. And they showed their "palpable excitement" for that by getting a whole lot worse.
And Blair had plenty of critics on his own benches, not least one Rt Hon Jeremy Corbyn.