Frankly I am mostly not driven by economic reasons, but for many everything reduces to that. So be it
Catalan GDP per capita iss above average than EU. Not being inside the EU? is not the end of the world. There are countries outside that they are doing just fine.
Then catalonia's debt. Catalonia's debt is minimal, 15% of that debt would be for catalunya if going outside Spain would be mutually agreed. If it is not agreed, the Spanish debt says "kingdom of spain". The creditors would go to spain, not catalonia, and most of catalan own debt is with the spanish government. Not independence agreement, means only 20% of debt over the GDP, the lowest in europe. You want us to accept the debt? we can talk about EU (that for other none economical reasons I am not dying to be in, frankly).
Then, if you put a border in catalonia, what happens with all the products that goes through catalonia in and out of spain? most of them passes through roads, rails and port of Barcelona. Sure spain can redirect it, and long run new roads and others infrastructures. Do you think is in their best interest putting borders there? and for France?
How you cn avoid catalunya to use euro? panama is using the euro, ecuador and el salvador didn´t ask permision to US. You just need to buy the currency and we already have it. EU, in case that they would be interested in that (that they would not be) would have virtually no power to avoid it.
And yes, the last part is true, Nobody said it would be easy, but Catalonia has the potential to not have any problems to succeed economically speaking.
And the truth is that at the end of the day, nobody knows what would happen, there are too many variables. It is true that economically speaking is better to be in a big block, but is true as well that spain is draining to much resources from our economy, so is a matter to know if it would compensate the loses of being inside Spain and EU in terms of trading with the resources not drain by the spanish state (recognized by the spanish government itself).