I assume the Brexit theory is that all that will be replaced and more from the RoTW. Even if it is (a dubious proposition)
Why so? There are 27 other countries in the EU, most smaller than the UK, but 170+ in the RoTW. While many of those 170 are tiny with no money there are plenty which are big enough to be good trade partners.
You're right, there is a lot we don't know about how things will play out when we're outside the EU, but frankly I'll take my chances on that. I think that staying in the EU and hoping everything will come up roses is an even more dubious proposition, and we have historical evidence to rely on for that.
I do find it saddening that so many people refuse to even consider that there could be a better future, and are happy to leave everything in the hands of someone else. It's almost a religious attitude, "God (or the EU) will provide, we should be grateful for what we get".
Sorry, but I don't believe in a benevolent God, nor in a benevolent EU.