Purchases sure on most chains (though not on e.g. Secret) but the chain isn't recording your logins via your address in most cases. You are just showing to the site that you have the key without making any transactions. So no, advertisers won't have much extra data on your site visiting habits that they don't have now.
In the same way tracking your location for sure can determine who you are.
https://www.nytimes.com/interactive/2018/12/10/business/loca...
Based on just a few transactions they can figure out that you're a middle aged guy, living in Nowhereville, Iowa, that's into samba, anime and drives a Toyota Corolla, etc. and then from that it's not hard to actually figure out who that person is.
Right now I use my country's currency, do you want me to go to a mix of currencies like I'm playing some sort of Android free to play game?
That's just adding complexity to my life for no obvious reason.
I dislike OldFi and want to contribute to a new, healthier system to serve people in general. But, short term, risk of DeFi is obviously higher due to its imaturity.
OldFi has been maturing for 100's of years. Perhaps we could argue 1000's. It'll take at least 20-50 more years to reach a similar maturity in DeFi.
By maturity I mean: people understand the risks and know what to expect.
If you are just doing login by registering your public key with a service.... then you are just doing signature-based-authentication that you could have done two decades ago.
1. Users understand passwords. While password stuffing is a concern, almost nobody actually makes product decisions based on rational security analysis.
2. Only a small number of users are concerned about centralized login via OpenID Connect. The privacy and account access weaknesses of logging in with Google or Facebook aren't concerns for the huge majority of users.
3. Creating and maintaining a key pair is less ergonomic than creating and maintaining a password.
#3 is now different for people who are regularly using blockchain systems so it doesn't surprise me that uptake is larger than now than in the past. More people have a Coinbase account than use PGP. But I am not convinced that people actually care enough to swap if they aren't already ideologically aligned with blockchain technology.
For #1: New people come to technology all the time. They don't understand passwords yet, so if something is better than passwords it can make inroads.
For #3: It's not clear to me why this is the case, besides just that it has always been the case. It's really un-ergonomic to create and maintain passwords. It just seems the alternatives have been even more so.
(Interestingly, I don't think Coinbase actually gives you a keypair you can use to login to stuff...)