For those not reading the article: the purpose is to have better customer engagement, not to affect security or privacy in any way. They have a fractured ecosystem (customer-wise) and it makes people's lives annoying. (It's also a missed opportunity to simplify business intelligence)
The benefit to you, the consumer, is deeper connection to the AWS ecosystem. Your contributions can be tracked on a dashboard and added to a virtual resume, so you don't have to list every god damn service and account you've worked with in your resume. You can more easily contact support across accounts and services, forums, etc. If you've ever gone, "shit, in what account/region did I use that one S3 feature before?", you could look it up in your global user history.
It's the same thing as having one GitHub account that lists all your contributions across orgs/repos. You can always create another GitHub account linked to another key/email/identity.
Solving this is a good idea, and can be done without much technical work, but it won't be, because of their business model.
This is what Google already does: all your "stuff" is linked to a Google account across all their products/services, because their bread and butter is knowing who "you" are everywhere, so they can make money off "you".
AWS doesn't care who "you" are because (outside of Amazon Prime) they don't make money off "you", they make money when you pay them. Very different business model. There's effectively no business case to track "you" everywhere, so they're not going to put in the work.
But actually, this could be solved easily with PGP keys and a database of databases. Add your public key to every system that AWS has (or they can add it for you, based on your email address). They can look up your general information in any public key server. And if they need verification of who you are, just send them a signed E-mail or file or something. It would be tedious to do this manually for every service, so they can architect some internal service to map public keys to internal services to hopefully get the human validation part down to just one time.