At first blush this looks almost like the Scottish independence referendum of a couple of years ago (which failed) except the Scots are a money sink for the UK government. A lot of government money goes into Scotland, little comes back in tax revenue. But from what I understand Catalonia is actually the economic powerhouse of Spain. Yet the central government has made it impossible for them to thrive independent of Madrid. For example they've made direct flights into Barcelona difficult, most of the time you have to go through Madrid. Same with shipment of goods from their ports. This hamstrings them while at the same time their tax burden is disproportionate to the rest of Spain in terms of collections versus expenditures on infrastructure and the like. They are essentially the bill payer of Spain.
Troops will march in before Spain gives that up. And that would put the EU into crisis. Hence a compromise where Catalonia gets some more autonomy is probably the most likely outcome at this point.
It’s rather hypocritical of the EUropeans that they violently supported Kosovo (Muslim) independence from Serbia while it’s shaping up that they’re going to support the violent suppression of Catalan sovereignty.