> EU is the worlds biggest single market
The US is the world's biggest single market, by a considerable margin over the EU now. It was larger than the EU before Brexit, now it's that much bigger.
US GDP for 2019 was around $21.5 trillion.
EU GDP for 2019 minus the UK is $16 trillion ($18.7 trillion with the UK).
That giant $5.5 trillion gap is big enough to fit the world's #3 economy (Japan) into. It's larger than France + Italy ($4.7t), or Germany + Spain ($5.2t).
Given the major economies of the EU haven't seen any net economic growth since 2006-2007, it's unlikely the EU will match up to the US economically in the coming decades. Over the coming 10-20 years it will persistently fall away, an increasing distant third behind the two superpowers of the US and China. And that's the good scenario and assuming there are no more exits from the EU. The US will become more aggressive about its trade imbalance with the EU, with the EU heavily dependent on exports and in a position of weakness with the US accordingly; and China will seek to compete with and replace the high value manufacturing they import from the EU (with most of the focus in both cases on Germany, their future is stagnation if they don't figure out how to move to a modern tech economy and fast). Germany will go through a very painful deindustrialization that they've largely been able to avoid thus far, as China goes after their high-end manufacturing and then exports those competing goods to the rest of the world far cheaper than Germany can.
France GDP: 2008, $2.9 trillion | 2019, $2.7 trillion
Germany GDP: 2008, $3.8 trillion | 2019, $3.8 trillion
Italy GDP: 2008, $2.4 trillion | 2019, $2 trillion
Spain GDP: 2008, $1.6 trillion | 2019, $1.4 trillion
Netherlands GDP: 2008, $948 billion | 2019, $902 billion
And that's of course pre-pandemic. Those are the five largest economies of the EU, accounting for 73% of the EU economy. They're going to see a likely net zero (or negative) economic growth for 20 years or more (inflation adjusted it's going to be brutal).