The two biggest problems with the 'better on the other side of the river' argument are that we are not sure it is true, but even if it is, we need to swim the river to get there, and could drown.

Or to put it less dramatically, all of the costs due to disruption during the few years Rain Dog mentions have to be recovered before you can even think of being better off. If, for example, during that time a long term contract comes up and because of the disruption it goes to another country as supplier, it may be decades before it comes up again so the business concerned (and hence to some extent the UK) is disadvantaged for all that period.

I thought Zoe Williams described it quite well in a recent article

So there emerged two discrete, parallel contexts for the discussion: one completely abstract, in which democracy must be upheld and sovereignty restored; the other completely concrete: who do you sell herring to if you don’t want to eat it, and where on earth do you have an Irish border? Never able to knit those two spheres back together, we had instead this jarring dualism, where we’d talk grace notes in Westminster (take control of our money, borders and laws!), and car plant closures outside Westminster, and those discussions never progressed since they were never informed by one another.

Whether the country thrives or not depends on the concrete: the herrings, the cars, the sheep exports, the services provided and so on. Sovereignty is only really relevant (in trade) to the extent it affects those.