Brain surgery is invasive, but the power that the surgeon exercises over the patient is bound to the context of the surgery. Side effects may linger, but the surgeon cannot increase or alter the side effects after the surgery. With Neuralink installed, the owner of the proprietary software that runs on it (because let's be real, there's no way they'd let you control your own brain) has indefinite read and write access to your mind. No thought is private; no action is your own. Everything that happens in your brain happens either because the owners let it, or made it happen.

There is a market for advertising-subsidized neural implants.

Think of the profit potential of being able to directly influence consumer brains patterns. It's a multi-trillion dollar industry, minimum.

The fear isn't that it's an option; It's creating a world where you can't afford not to. Much like how modern life requires internet participation, the idea of a potential world that requires neural implant participation and the economic incentives described above is viscerally horrific.

As mentioned by another comment. First episode of season 7 of black mirror paints a really scary and very much likely outcome of this type of tech.

Test animal living conditions?!

Last I checked they where probably treating their animals the best out any research lab world wide?