Spain would have had beautiful cathedrals even without its colonial empire.
That´s a good point, especially as most of Spain´s cathedrals were built well before Europe even knew the Americas existed (Baroque façades and chapels are a different matter of course)
I have a notion that thousands of people died during the construction of most of these great edifices
Thousands may be an exaggeration, but given that construction is even now one of the most dangerous industries, maybe not a great one. But if it had been that dangerous, they could have just walked off the job, they were free employees, not serfs or slaves and quite well paid in some cases (e.g. Master Mateo, William of Sens). This raises another issue: economic stimulus. The building of cathedral must have given a massive boost to local business, rather than wealth consuming, cathedrals may have been wealth generating.
I think this is a rather simplistic, even naive, way to frame any discussion on this subject.
Not really, it is simple, but most ethical questions are simple, it is the answers that are complex. We enjoy and benefit from visiting religious buildings, are we benefiting from someone else´s suffering and hardship? If 'yes', that is basically wrong unless there are mitigating circumstances, and it is a big 'if'.
The Americas were yet to be discovered and plundered, Jews were unwelcome but not officially persecuted before the reign of Ferdinand and Isabella, Muslims were actually employed on many churches, local and itinerant artisans were employed, businesses and ancillary industries such as quarries and forestry were busy, so it was a win-win, no? Well not exactly. Every society generates wealth, but that wealth is not necessarily spent for the benefit of all its members. The decision makers in medieval times were the monarchy, the nobility, and the church and monastic hierarchy. They were not the wealth creators. They spent the wealth on grandiose and often self-aggrandising schemes and even worse on pointless and petty warfare, not to mention the Crusades.
So how do I feel when I stand inside Burgos cathedral? Pretty impressed, actually. The skill and craft is overwhelming, it is a stunningly beautiful building, as are all medieval cathedrals more or less. I spare a thought for the hardship of the common people, which I can't redress, but mainly I respect their achievement and artistry in creating this marvel.