Yep. A common European first language would start a process which would result in a terrible loss of local cultural tradition. Cultural suicude for the common good has been suggested before, but I can't imagine that there would be a lot of people who'd agree with it.

I disagree, I live in Denmark where the majority speaks very good English. There's no shortage of Danish media.

On the other hand, economic suicide is assured by insisting on a high standard of living while doing everything possible to prevent innovation.

As a Swedish-speaking Finn, I can confirm that a culture can end up on the decline even if there's powerful legal backing to preserving it (including forcing the remaining ~90% of the population to learning Swedish in school). Incidentally, I'd say there's a large correlation with English being more and more accepted in Finnish society, but I suspect the actual root cause simply is that cultural production is mainly taking place in the first language (Finnish) and that it leaves less for the others.

The same would happen if you managed to get the entire EU to agree to switch to one single language (remember, it's likely that it would be French or German instead of English). As a result of pushing adoption of the new first language, it would mean increased cultural output in that language, and a reduced output for the others. After a while, the differences will start showing.

I've lived in Denmark for quite a few years and I agree that people in Copenhagen and the other larger cities wouldn't notice that much of a difference to begin with, but remember that this would mean a cultural shift that will take multiple generations. Ask the German speakers in southern Jutland in another 24 years, and compare how their culture thrives compared to how they did before the war and you will probably have a pretty decent view on how the situation would be for many, many of EU's minorities 100 years after the switch to a single European first language.

I'm sure that some wouldn't mind other's cultural suicide, however.

Definitely not. There's probably no shortage of people promiting their own culture's demise either.