I legitimately don't think delaying could conceivably help either the Lib Dems or Labour. This is the moment of the Tories' greatest weakness- Johnson's just had to break his one big pledge, he hasn't passed his Brexit deal, Farage is stoking the Betrayal of Brexit fires, and he hasn't been able to properly set up his People Vs. Parliament narrative. I fail to see any scenario where the Tories aren't in a stronger position in a few months time- particularly given that the numbers are almost certainly there to pass the withdrawal bill if the opposition chooses to sit on their hands.
Yes, Johnson holds all the cards, and yes, the polls are heavily in their favour. That's just the reality of the moment, and there are no signs that that will change in the foreseeable future- now is the time to roll the dice.