For myself, it isn't the individual repersonalisation I'm concerned about.
It's the mass munging of the data of the sort that Facebook did, where it took members' likes of music and posts, and and used it to segment the population into psychometrically aligned groups it called "universes". This data was then used by Cambridge Analytica to target universes, for purposes of political manipulation.
Retailers use the same Facebook universes to try to sell you more stuff.
Facebook also collect a lot more data on you, eg when you visit third party sites like MN that have the Facebook icon, and also using "device identification data" to recognise your mobile phone or laptop when you're on their site. It also, of course, asks for your phone number, and can potentially use that to identify you across a lot sites.
Facebook isn't the only company doing this, of course, just the most well known (and the one that got caught). Lots of sites now collect and trade in our data, and there's the computing power to make use of it in a way that didn't exist when the internet was young.
Oh, and if you don't have a Facebook account, there's a strong suspicion that Facebook is creating what's called a "shadow ID" for people as they move around the internet.
This is what's going on in the Big Data world.
I don't know all the things our medical data will be used for, but I'm pretty sure that some of it I won't be happy with.