And, like in all the other case about ilegal state aids, the EU can go back 10 years since the start of the investigation
Say I'm a balling billionaire, wanting to invest, and politicians in an EU country are willing to attract me by giving me a very generous tax break. There is no reason to say no. However, in this particular case, if ruled that this is state aid, a country gets rewarded with additional tax revenue for doing a bad job / acting like an amateur. They need to find a different punishment mechanism, because in its current state this just incentives countries to introduce half assed laws and systems.
Second, while such a ruling does indeed fix the situation, there are usually plenty of alternative ways of structuring a transaction to achieve a similar tax rate. Why would I ever bother making a deal with a country that has "screwed" investors over in the past?
Claiming ignorance just makes you look stupid.
People who get too clever with that sort of thing are why there are Federal prisons with golf courses.
Given the climate around Brexit and the fact that Ireland is fighting back on this ruling with Apple, it is astounding to me that you think this is "fine".