There isn't really a line because "a decent VPS" is exactly what EC2 is. You use it if you * are averse to the kind of risk that comes with being tied to a sma

There isn't really a line because "a decent VPS" is exactly what EC2 is. You use it if you * are averse to the kind of risk that comes with being tied to a smaller provider, and/or * are not averse to the kind of risk that comes with being tied to a provider who cares not one whit for your well-being because your business is insignificant to them, and/or * actually have a use for, and staff with the expertise to take advantage of, any of the million proprietary bells and whistles that AWS provides, and * are not averse to absolutely abysmal UX. Discourse falls mostly into category 3 above; as Sam enumerates they are taking advantage of a bunch of various AWS offerings and have split off several of their supporting services onto their own instances. Those things can be built up gradually; you don't just wake up one day and decide to become "Enterprise" and watch all your costs increase by two orders of magnitude.