Take a look at the following graphic:
The eye-catching thung is not that China has made progress - you would expect 1.4 billion people to make progress after they joined the world trading system (WTO).
The eye-catching thing is that the USA has held it's own while the EU has fallen back. This is despite the EU (including the UK) having 500 million people, while the USA had 330 million people.
What has happened in that both the USA and EU have outsourced manufacturing of cheap stuff to China. But the US has replaced those jobs with new industries - the whole tech thing (Google, Facebook, Twitter etc) plus fracking (the US is now self-sufficient in oil), plus improvements in agriculture and other areas.
The EU hasn't come up with anything to replace the outsourced jobs, and they keep passing regulations to stifle innovation - the latest idiocy being the link tax.
After the UK leaves, with it's 65 million people and $3 trillion economy, the EU's position will get worse. The UK has already carved out new industries - it is now dominant in higher education, with elites from all over the world sending their children to UK universities to get an education. The education sector is thriving enough that it can afford to raise fees for non-UK students without detering applicants or the universities having to lower standards.
The UK also has a massive media sector, led by the BBC, alongside dozens of independent media companies that produce award-winning shows (The Crown is one example, produced by Left Bank Pictures and shown exclusively on Netflix). Downton Abbey, produced for ITV and then licenced to Amazon Prime is another.
The EU doesn't have anything apart from car companies that made a bet on diesel and lost.