It really isn't
No-one at all is saying 'We'll be done for' or 'we'll never do any trade again' They say there will be a period of uncertainty (that markets & business never like) and it will take years to negotiate our own individual trade deals with other countries separately.
So there will likely be a short term hit to our economy and we will end up with tariffs and administrative paperwork when trading with other countries (like we already have with non-EU countries), but now including the EU countries.
It will be a bit more of a burdern, but it wouldn't be the end of the world.
I think we just take for granted now when we buy or sell anything across the EU now, how simple it is and even though filling in duty/import/export paperwork is a pain in the ass, it's not going to stop people trading with us.
But, when multi-national companies have the choice of where to go, maybe the extra level of admin might sway them, especially if the sweetners being offered are similar to elsewhere.
Probably all the above is true and it will or may hurt the UK for a number of years post exit. But what investors and business want is a secure political system, a stable currency and low taxation. Each of the above can be provided by the UK.
Our situation is much improved from pre EEC and we have done well from the trading within the EU. However we now should reclaim some ability to sell our skills, our intelligence and our capabilities worldwide and compete with the EU where we can but cooperate where we can't.
It is down to politicians to be cleverer, not so flippin' lazy and to gain some original thinking as to how we sell UK.biz to the world.