I agree with all of you that this tax and especially its purpose is stupid and the only winner can be the 2-sector business model airlines (i.e. Emirates, Gulf Air & Co.). BUT the tax will come and everyone flying to EU airports will have to pay it. And if the companies won't pay directly, governments will mandate, that the airports are responsible for its collection. That's for the enforcement part of it.
Legally an emission tax for flight between EU and non-EU is IMO very problematic. Since due to ICAO agreement jet fuel is supposed to be tax free the EU move is a clear violation of this agreement. There can be no doubt that emissions are linked to the fuel consumption so this move is kind of like tax avoidance (just the other way round
) which is not allowed. And if the ICAO explicitly confirms that such a taxation is not conform with its statutes the EU will have to comply or unsign the ICAO statues. However if the emission tax fails, the EU will find a new way to tax. E.g. mandating a levy paid at the airports depending on the weight of the aircraft.