Warrants and subpoenas are inconvenient. They are targeted and may take time to be issued. And observing from the outside, US as a nation is still years - like

Warrants and subpoenas are inconvenient. They are targeted and may take time to be issued. And observing from the outside, US as a nation is still years - likely decades - from being able to stomach general warrants. But you are already well on your way towards something like that: reverse warrants are now a thing. Law enforcement in US has already issued a warrant for recorded information on all persons who were near an event after the fact. The logical conclusion with brain data archives will be to issue warrants for full mental activity history for X years on people who have had thoughts on a given unwelcome subject. But even then, warrants are inconvenient. It'll be much simpler and easier to just buy access to the data in bulk, all the time. That avoids the awkward questions on storage, access and oversight.