The Labour Party needs to look in the mirror with this one. Johnson had to quit because too many of his own people would no longer back him No matter what his mandate, agenda or intention was - it was time for him to go because he would not be able to do it without them.
Same thing would be true for the Labour Party. If Jeremy Corbyn had been elected prime minister, he might have had the most wonderful, radical, socialist agenda but if his own people did something similar he would have had to quit as well.
You can say the Labour Party does not have enough radicals or socialists and that may well be true. However it is not a question of where you are on the political spectrum or what you want to do. Without the serious support from your own side - none of it is going to happen.