"What made the powers that be believe introducing a common currency a good thing in the first place, "
Well, just one example from my own experience:
Back in the late 70s - early 80s when I first got into computers, there was a thriving UK software industry. Of course, all software houses produced stuff for the local market first and then tried to expand into the rest of the world.
The local market for UK companies was tiny compared to that of the USA, which meant that a US startup had room for enormous expansion before it ever need to worry about localisation (foreign languages, currencies, measurement systems, etc., etc.). Consequently, any UK company that wanted to compete on the world stage found itself going head to head with competitors with 5, 10, 20 times the resources.
Now think about all the software that you use regularly. How much of it comes from European companies, and how much from the US?