It's great that the EU passed this. A couple countries like Denmark have been working towards a 2020 goal for a couple years. Hitting that goal is basically impossible but I'm glad they're trying. The obstacles are not technological. They're organizational and economic. In the "publish or perish" environment of contemporary academia, researchers have to publish in whichever journals are most prestigious in their field. This has a direct impact on their career's trajectory and their chances of getting tenure (or equivalent).
If the prestigious journals in your field are not open access then you have to choose between the option that benefits your career (prestigious journal) and the option that benefits more people (less prestigious open access journal). In some fields this is not a problem because you have prestigious open access journals like PloS, but in other fields there is no prestigious open access option.
When we had only a few countries requiring open access, there was an extra dilemma for a researcher in that country -- do I stay in this open-access-only setting, publish only in open-access journals, and risk losing my competitive edge in the international job market, or do I take a job in another country that will let me publish in the most prestigious journals regardless of whether they're open access.
Making this requirement EU-wide radically decreases that dilemma. It also creates pressure for publishers to make high-quality open access journals in every field.
Something that rarely get mentioned in the coverage about open access: most OA journals recoup their lost subscription revenue by using a pay-to-publish model. This puts the financial burden for publishing on the researchers, which creates its own set of incentives. For example, some argue that the pay-to-publish model has decreased the overall quality of papers in academic journals.