I wish it was as you say.
The address bar cannot be taken into consideration for Search Engine Optimization Analysis.
what matters is the domain.
Google sees steemit.com as an unique website
if you post some spammy article the penalization Google gives affects all the community.
That cannot be true because places like wix.com or wordpress.com are not penalized and they have a ton more crap sites on their domain. They have free websites attached to their domain with literally nothing on them and they are not damaged by it. Google's algorithm is very smart and can understand the separations between each username. Now don't get me wrong. Good posts and informative posts do benefit Steemit but not exactly in the way you think.
wix and wordpress are using subdomains.
steem doesn't. so, yes spam on 1 page penalizes how other pages are ranking on search engine results.
google algorithm is smart and could do it, but actually the domain is what matters. there is no distinction between username. do you have a spare Idea how much computing power they should use to achieve what you say!?
if you post something on your blog it gets high because of steemit.com
if you post some spam google penalizes all the domain not only your @joeyknowsbest folder