I don’t know what financial contribution Spain makes to the Eurozone compared to what it receives so if anyone knows this it would be interesting 🙄
However, IMO, Spain has benefitted enormously with infrastructure improvements, you only have to look at it’s super new roads, we drove on these to avoid Madrid when driving from France to Malaga and they were superb. Not only smooth and without potholes (brand new), but virtually traffic free, an anomaly really, the best roads but without traffic! Apparently the UK has contributed loads of money to Spanish roads through it’s payments to the Eurozone, but the UK’s roads are now some of the worst in Europe. It’s only in towns like Ubeda that the roads were poor.
So, I would expect Spain not to want to leave the gravy or gaspacho train, mind you if it did, it could devalue, and possibly revamp their tourist and property industries 😆
The old motorway roads out of Madrid are still pretty poor, but they have built new toll motorways to provide new alternatives to escape the city. It sounds like you might have taken the M50 which is part of a new road scheme to allow people to cross Spain without having to deal with Madrid.
Generally the problem is that you have cases where for example Spain receives money from the EU to improve its roads “because it hasn’t got the money” while at the same time Spain somehow has the money to build the best metro system in Europe for Madrid (the metro stations and trains in Madrid even have TV screens feeding you council propaganda). As with any system based around some central government handing out money for certain causes, the causes for which people receive money never get resolved.