As long as the entire idea is to ship silver from Potosi abroad, Buenos Aires works in a similar fashion as having a port in Bourdaux to export Polish goods, but with a barely populated region in between.
Except that Buenos Aires is directly downriver from the Bolivian highlands along navigable waterways, so not really the same thing. Also Argentina might have been sparely populated but what population it did have was largely conentrated along the banks of those rivers so it's not like they'd be going through a desolate wasteland. If anything it's like shipping stuff from the Alps along the Donau, which is the same distance and considered completely normal.
So yes, initially it would probably revolve around Bolivian silver, exporting that while providing all the other goods and services that ships would need for their return journey. Which is what allows a local economy and market to develop and expand.
That's why I think development needs to come first: start sheep ranching to provide wool for textile mills and add additional economic activities: cannabis, ship building, turn yerba mate into an export product, build steel mills in Paraguay. Once this is taking shape you now have something worth defending.
You're not going to develop an export economy without being allowed to export stuff first... Furthermore you can't just project modern industries over historical settings and assume it would work. Argentina is a good place for sheep ranching for example, but it's very unlikely that Spain is going to allow the export of sheep wool because its production was far too important to the economy back in Spain. The same is likely true for cannabis/hemp, which was produced in Catalonia, Valencia, and (to a lesser extend) in Aragon.
The kinds of timber needed to build large seagoing ships can't be found anywhere near the Rio de la Plata, so they'd have to be transported over vast distance. And from what I'm finding there was little to no overlap between iron deposits in Paraguay and the places that the Spanish actually occupied, were they even aware of any noteworthy deposits in the region historically?
And there simply was no interest in yerba mate in Europe, and growing more of it doesn't mean merchants will actually want to buy it and take it back to Europe. Also wasn't yerba mate production controlled by the Jesuits?
Edit: quite frankly I'm not even sure how this is debatable, historically it was opened up to trade before there were any noteworthy local industries and it did actually lead to a massive growth of the local population and economy. Potosi silver was shipped downriver to Buenos Aires after the Bourbon reforms. Salted meat was produced to provide provisions for ships and for export to slave colonies, which simultaneously spurred the production of cattle hides, etc.