The easiest way to understand why the WA+TP is a disaster is to imagine the UK being made to walk the plank, like in a pirate film. The water below the plank is boiling with sharks.
The WA defines what the plank is like, and how much of ship life will continue once you've walked out onto it. The transition period defines how long you have on the plank before you have to make the leap.
So under the TP, the UK will be standing for 18+ months at the very tippy-tip of the plank, staring down at the circling teeth. There's no way back onto the ship - that way's closed off by having left the EU. So we're just swaying and shivering and watching our doom eyeing us up.
Life on the plank's not too disastrous. It's definitely not as great as being on the ship, but it's bearable.
But as soon as the transition period ends, the plank gets jerked abruptly from under our feet, and it's into the sharks we go. Chomp, gulp, the end.
So we have 18 months or so to work out how to change our situation so that we avoid the sharks. Without getting back on the ship, because that avenue's closed off (we would have to apply via Article 49, way too long and complicated to be of any value during the TP).
But if MPs spend the TP bickering over what Brexit should look like, just as they have done for over 2.5 years already, we'll get nothing. The full horror of no-deal will hit us at the end of the TP, just as it's threatening to do on 29 March 2019 because we haven't reached a deal yet.
The biggest danger with the WA isn't the WA itself, but the fact that it makes MPs feel "safe" enough to vote for it. It feels like progress, because it "delivers Brexit". But in reality it seals our fate. Right now, we could still retract Article 50 and remain. Once we're out, that escape hatch goes away. We will have no treaties, no agreements, no trade deals, only a pale copy of the effects of them. So we will have to rebuild our international standing (something that has taken 40+ years to establish) in 18 months.
How likely does that seem, honestly?