Advertising? Have some articles be part of the bundle as a "preview", and most articles behind an individual subscription?
This idea of building your own newspaper sounds kind of interesting actually: a mix of authors you've picked and an automatically curated content from other authors to help you find new authors.
The social networks are offline. Universities are great at open collaboration, it's just not a kind of online collaboration where anybody can walk in and partake. People spend a lot of time on a different type of communication: going to conferences, attending seminars, and meeting one-on-one. Why should it be online?
Also keep in mind that a lot of mathematicians are also older, less tech-savvy. This might change in the next decade or two!
I've heard of mathematicians being frustrated by physicists who use fancy stuff without full understanding of the underlying principles, and sometimes they just manipulate the notation without fully justifying what that manipulation means (e.g. you may have an integral operator on some space, and you might just swap an infinite sum and that integral operator, but that requires a justification that you might omit).
Pitting code review against trust and responsibility is just disingenuous. The point of code review is not that you distrust others. It's that everyone makes mistakes and often someone else having a quick look over your changes can come up with some really cheap wins that are safer, more maintainable, faster, etc.
Doesn't seem like there's any current vector versions of that style, but you could make one yourself using Maputnik or similar; you'd probably want to start with OpenMapTiles and maybe even one of the pre-existing styles for OMT like OSM bright.
