So the biggest threat to Substack is unlikely to be the Twitter-centric political battles among some of its writers. The real threat is competing platforms with a different model. The most technically powerful of those is probably Ghost, which allows writers to send and charge for newsletters, with monthly fees starting at $9. While Substack is backed by the venture capital firm Andreessen Horowitz, Ghost has Wikipedia vibes: It is open-source software developed by a nonprofit.