It's just a totally unknowable situation. The uncertainty of what the final arrangement may look like - businesses don't like uncertainty. The British government needs to start the countdown as soon as they can, to get past this phase.
OTOH, the EU is going to be nasty. They must punish the UK, because otherwise the next 2-3 countries that might leave are already lined up.
Switzerland is in a similar boat. The voters here have said "enough" to immigration, even from EU countries, and required the government to put a hard limit in place. This was two years ago - the government has failed to do anything yet, because the EU basically refuses to negotiate. If we put border controls back in place, we lose all other agreements with the EU. They don't dare open the door to border control, otherwise all of the EU countries will also do it. Some, like Austria, pretty much already have done so.
The EU is playing a losing game, in the long term. The open-borders, pro-immigration politicians (like Merkel) refuse to admit that they were wrong. They refuse to acknowledge that national borders have a purpose. That unlimited immigration is the same as cultural suicide.
Their stubbornness means that the rebound will put extremist parties from the other side of the spectrum in control. In five years, we won't have just border controls (those are inevitable, at this point). By the time the reigns of power can be ripped from Progressive hands, the resentment and fear will have grown to such proportions that even peaceful, integrated immigrants will face persecution. Good intentions are, as usual, paving the road to Hell.