They can't. It takes decades if not more. There was a huge investment in UK, UK has unique competences and capabilities, why did you think they export so many services and their universities are behind american only. UK is a huge economy and a huge market even without closer EC ties, most of multinationals will stay. Take Russia for example, even with sanctions, decreasing economy and other troubles just a little number of foreign investor and companies actually left and closed their operations there. A huge number remained, many even continue to invest more etc. Arabs, USA, Asia will still invest in UK.
To suggest that the country with golden triangle, with great capabilities in life science (i can speak from experience on that one, since i am in biotech, pharma) will be left with chips and fish is beyond foolish and uneducated actually.
EC gave UK a good conditions sure, but other countries also had them, yet their respective economies were nowhere near as strong as UK, France or Germany.
Those who think EC will do just fine without UK and that UK is no more significant as Slovenia/Poland/Hungary/whatever just don't get international trade and has very little idea of how multinational business is actually done. Just look at the numbers, how much trade is done with UK. Look at Germany-UK connection. Germany won't export these goods to Poland or Slovenia, they just can't pay for it, nor do they have something to offer Germans. Just get real, do you think Eastern Europe will buy as many german cars as UK does?
UK is a huge, wealthy market and no way EC will cut the ties completely. So the deal will be on the table, it may be a better deal, it maybe worse, it may be the same. But some sort of compromise would be struck.