Most of the answers here are dwelling deep into the text readability & legacy research (I am not saying that there is anything wrong about it) about typography. However, I'd be more concerned about the purpose that text serves.
In the wake of mobiles & complexity reduction, Big, bold, centred typography with short line length seems to be the first choice of the industry. This behaviour seems to be driven more by the dynamic business requirements (I'm excluding the obvious ones) such as:
Centre aligned text plays well with multimedia content insertions.
Negative space/minimalism is a widespread UI pattern now and is part of all the major design frameworks to an extent (flat design, material design and all new systems draw something or other from minimalism).
A centred text plays really well with varied screen sizes, combined with (even) larger text sizes, it creates the best mobile reading experience.
Talking about design perception for humans, equally distributed white space is known to increase user comprehension and allow users to create mind maps easily. (https://www.fastcodesign.com/3046656/why-white-space-is-crucial-to-ux-design)
In addition, it's all the matter of the objectives of the design more than anything. "Providing ultimate reading experience with implementing all UX laws" is rarely the objective. sometimes a page is just meant to share a piece of information which caters to a targetted audience or for sharing information at the individual level without much of a business objective.
Paul Graham's site has a lot of white space and it still a regular left to write text orientation. but it's also not mobile optimised, neither it is the most aesthetically appealing designed page. It's there to serve a purpose and it's doing it greatly for while now. That's one of best learning for me as well. First & foremost reason behind any design is it should get work done, everything else is secondary.
Here are two screenshots for ya'
Now, they both serve their own purposes, however medium caters to a large audience and thus requires an aesthetically appealing interface with compatibility for all screen sizes. Also, it caters so a very wide audience. On the other hand, paul graham's page is more like a place where one would reach when he is looking for some expert advice and it serves that purpose as well.
What are your business/personal objectives? who is your target audience?
Discussing anything before putting these two questions in context will not yield you any good results.
PS: I'd always go with minimalistic, centered (but left aligned) text with a short line length & relatively higher line hight. why? because that just makes more sense in the time we live in ;)