Having lived in Madrid for a number of years and travelled frequently to and from London - there is always a huge amount of passengers on both IB and BA flights between the hubs that are connecting onto flights to central and south america. It obviously seems that this is now also the case going the other way. IB's service to north america has always been poor with only ever one or two flights a day to the NY so they are obviously filling the planes up going to LHR as well. They also no longer have competition on the route and have kept the full fares very high since BMI pulled out of their half hearted attempt to serve the route back in the mid 2000s (which they used to do with many routes).
I am sure that the SA network will always be a good source of income, and indeed BA even dropped certain direct routes when they went down the code share route with IB - such as Bogota and Caracas. So the codeshares work both ways.
The main problem with Iberia has been and still is resistance to change with the workforce, and the decimation of their domestic market by the low cost airlines and more recently by the opening of the AVE rail line between Madrid and Barcelona. When you can get on a train in Atocha and get off in Sants in not much more time than it takes to get from the centre of both cities to the airports and get through security etc - it is easy to see why these days an IB aircraft is seldom seen in BCN. To put the blame for all of this as BA's door is a plane farce of Christina Fernandez de Kirchner proportion.