Horeshit. There is nothing advanced about storing only the data you need. However, if you've been hoarding like crazy and weighed down with technical debt, and haven't used the last two years, then yeah, might be hard.
Yes, it's a new law, yes, in practice it will be defined by judgments. How is this different from any other new law, other than this one impacts IT harder?
And let me guess what the alternative is: do nothing.
Consider also that European law is different to US law. We draft laws and contracts in a conceptual/abstract way, whereas in the US where everything has to be exhaustive, explicit, and over-worked; just in case anybody dare sues.
GDPR recognises one-size-fits-all won't work. Yes, that means it has some vague terms. Yes, you might have to show you thought about the implementation, and that you erred on the side of privacy.
It's funny how everybody always talks about the maximum fines, not the other sanctions that the GDPR can impose. Guess that's just more sensational.
Compliance depends on how well your understanding of of a bunch of fuzzy terms like "legitimate interest," "level of security appropriate to the risk," "necessary in relation to the purposes for which they are processed," "no longer than necessary," etc. align with 28 different regulators and judiciaries. That's as far from "trivial" as it gets. Bozho.net is not a lawyer, not your lawyer, and even if here were your top-tier lawyer specialized in data privacy he wouldn't have a clue what courts were going to take these things to mean in the context of GDPR, because there aren't any judgements yet. Security and minimization standards are also about "taking into account the state of the art" - do you know what the state of the art is, and is your organization capable of implementing it? An entirely plausible outcome here is that only the most advanced engineering organizations have technology that meets these standards.