I have serious doubts about how they came up with that number. Data captured once can be stored in a data warehouse and analyzed and reused in many different ways for analytics and reporting, so I am not sure how they estimate that 90% of data is never used again (unless, of course they meant that it is not pulled up again on the frontend application side, which would still make no sense at all).
At our hospital, they have replaced the inpatient electronic medical records system at least 3 times in the last 20 years, and our data warehouse, which has been around for more than 15 years, contains a large percentage of that clinical data from the different (current & historical) systems. A lot of this data is still used pretty actively for retrospective research, recruitment of patients for clinical trials, operational and financial resource planning, forecasting, cost-accounting, etc. In other words, at our institution, most of our data is used all the time, but for different purposes.