You have NO idea how the economy will be in 2019, 2020, 2021.
You have NO idea of the trade deals we may do elsewhere and the knock-on effect for our economy.
You have NO idea of the investment there will be in the UK after Brexit. Right now, it's very healthy, even with Brexit looming and not finalised yet.
Most people have more faith in the UK than Project Fear doom-mongers.
You are making predictions and they are all catastrophic, because you are a Remainer.
You like to think life is static and that what we have now is set in stone. In terms of the economy nothing is guaranteed and experienced forecasters appreciate that. The governor of the Bank of England got it wrong. His gloomy predictions didn't come to pass and he had to admit his misjudgement.
Yes, it's wise to try and anticipate the future, and it is wise to plan for the future. Just as the future isn't guaranteed, neither is the economy. We may find ourselves in new markets with a booming economy. That prediction is just as likely as yours, but sour grapes keep the doom-mongering Project Fear Remainers chuntering away about how awful everything is and will be, forever more.