This is a valid concern, but I think you're overstating the case here.
The issues with exporting data outside the EEA are already widely addressed in business terms and conditions by explicitly disclosing that it will happen. For better or worse, that is all it takes to escape the data protection regulations in most cases. Schemes like the old US Safe Harbor and its successor mostly just reduce the need for prior consent, and they're on similarly shaky ground when it comes to government monitoring, and still no-one is going to do anything about it because no-one wants to seriously undermine diplomatic relations between the EU and US. If all of this were not the case, EU businesses would not be able to use services based in places like the US, and the EU's tech industry would be dead.
It seems highly likely that a similar arrangement would be made between the UK and rEU as part of a Brexit deal or soon afterwards. There is little if any advantage to either side in not doing so, and a lot of potential damage to businesses on both sides if they don't. But in any case, there is probably still enough demand in the UK alone for a dedicated data centre even if certain rEU-based organisations would not be able to use it to store some information and consequently there was demand for a data centre in the rEU as well.