Correct to be concerned
I think that the most obvious issue will be that data collected and held in the EU will only be accessible by parties that comply with EU laws and regulations - the EU will not allow the UK to access and use that data under conditions that would be illegal within the EU. Similarly, data collected under UK laws and regulations, if they are incompatible with EU laws and regulations, may be illegal and inadmissible in the EU.
Then there is the cost; if the UK is not contributing to the cost of collecting and holding data collected in and by the EU then it will have to pay to access that data.
The Dutch estimate of ten years may prove to be optimistic.