Consider a school.
Current government advice for various data held is published in a nice compact table with type of data and years you need to keep it by law.
Some of it brings the limit up to legally requiring storing certain personal data for 25 years after they were last a pupil. And, no, you can't anonymise it.
As such, "right to be forgotten" for many such pieces of data is basically non-existent.
Even financial records tend to hover about the 4-7 year mark for even the smallest business, and no, you can't just anonymise them (the taxman may have something to say about that should you be audited for, say, VAT or income tax for private contractors as in IR35).
The right to be forgotten is a way off for most people and requests handled on an individual basis. But GDPR hasn't really considered it in terms of practical solutions.