Rack is cool, but the modern approach seems to be more decoupled web servers. You rarely see multiple business-logic Rack servers in a single process anyway tho

Rack is cool, but the modern approach seems to be more decoupled web servers. You rarely see multiple business-logic Rack servers in a single process anyway though; Rack servers outside your primary business logic tend to be used more like Express middlewares in Node (which are very conventional at this point). And the standard entry point for a NodeJS package is "whatever is defined as the main property in your package.json." Do otherwise at your peril.