Is there that much to negotiate really? A lot of EU laws are set by EU and implemented individually by the individual states. A Brexited UK is not obliged to repeal any of those laws, but can repeal / amend them on a case by case basis depending on what they want, without this being impacted by the 2-year Brexit deadline.
With regards to cross-border stuff - ie movement of goods, services and people, the UK has already committed to close it's borders, and EU is already committed to close down free trade if free movement of people is blocked, so negotiation on the main points is limited, it will be down to the fine details. If no trade deal can be negotiated by the end of the two years, UK will just be out of EU and legally within the EU have the same status as Somalia* or South Sudan* . That would be bad for EU, but absolutely terrible for UK.
Any deal that the UK can put together is far far better than no deal at all
* Just examples. I have no idea if these countries actually do have any trade deals with EU that would make their legal situation vis-a-vis the EU actually better than a post-Brexit UK
Biting the hand that feeds IT
