There's over 60 years of EU legislation, going back to the Treaty of Rome. It's been amended but not revoked. Same goes for the treaty of Maastricht. [See its 'in force' status on EUR-Lex]

Indeed, wow!

I don't know. There is a big bang next 29th March unless there is at this point an extra ordinary effort put in to avoid it. All treaties, agreements etc come crashing down at that point in time. This is not like most deadlines, which are really only a hint at a date by which which time it is hoped to start negotiations, and then really only start negotiations if something else more interesting doesn't come up in the mean time. This date next March is a different beast altogether which people aren't familiar with. This date is absolute and there is no easy way to extend it. Any deal to avoid this needs to be agreed by EU commission, European Parliament etc and probably most difficult of all it needs to be agreed by the head bangers in the UK Parliament. All this needs to be done before March.

There are only three options on the table now.
1) Extend the duration of A50. Technically this is straight forward, but politically difficult.
2) A custom negotiated deal. Extraordinary difficult technically to organize and is turning out to be all but politically impossible. Besides we seem to be out of time.
3) Crashing out.

Now, obviously if option '1' is taken it will be spun differently. It will be smudged and fudged over to try to give a face saving way out of the situation we find our selves. This though requires everyone to play along. Everyone knows it's a fudge but no one calls it what it is. This fudge requires good faith on everyone's behalf. The UK Parliament though is deadlocked and unable to function at present. There are a large number of its members who seem intent on watching the country burn. There are also those that are doing everything they possibly can to poison relations and seed mistrust.

Two years ago I thought there was a negligible chance of a crash out happening. Now I'm betting it's the most probable option.

I would agree, except companies have to take note of their supply chains and orders now, for the future. Airlines are going to be a real obvious indicator. If there's no deal in October, we can start to find things unraveling this year as everyone gears up for March 29.

It won't be just airlines. Long haul sea transport to and from Britain will have to start plannining soon too.

One thing I forgot to add, the biggest risk of disruption after Brexit is going to be from jobsworths who will query every single process that they carry out and will not do anything with express instructions from their management, regardless of the consequences.

Think of traffic wardens who will ticket a car who's driver fas just dropped dead of a heart attack.

The main difference I see between the GFC and Brexit is that the GFC was everyone in the West's problem. They all had a vested interest in sorting it out. Is it in the EU's/US/China's stategic interset to dig Britain out of a hole?
The GFC only affected one part of the economy, the banks. Brexit is all parts of the economy, it also affects defence, education, transport, justice. That's a lot of late nights, they'd want to be getting started last year to hit March '19.
Brexit is a regional problem, mostly Britain and Irelands with it's ramifications lessening as it radiates out. I can't see the US power structures sitting through the night like it did for the GFC to pass rules to allow Britain to trade as it does presently under the EU. Will the Chinese politburo?

If they want to avoid chaos, they will.

But who is "they"? If this was just the UK and EU in the mix, I'd agree with you.
And WTO MFN status hangs over all of this. If the EU or UK decides to break international law by allowing goods to continue to circulate after March 29 (without a transition period or FTA), as they do now, all other nations in the WTO will land on them like a ton of bricks.

The options to avoid chaos are massively constrained by the change of status of the UK into a Third Country, and the expectations of all other WTO nations as to how they will be now be treated by the UK concerning UK imports. Tariffs are one thing, but I understand (but may be incorrect) that MFN also applies to sanitary and phytosanitary measures. So both sides have to put in place measures that don't expose them to WTO litigation by any other nation. Outside of a transition period or FTA, the EU has to then strictly treat the UK as a Third Country.
That is, if Trump doesn't decide to try destroy the WTO first. This isn't unthinkable anymore. God knows what would happen then.

Given the UK does a lot of trade with the rest of the world under EU trade agreements, what will happen there, we are all aware of how the EU will treat them but by the sounds of it the ROTW will treat them the same if there is a risk of WTO litigation on anything other than WTO rules without separate trade agreements? I've read nothing of trade negotiations with the ROTW.