I believe though, that what you actually need as a big or small company, is good game first and foremost; the engine is secondary. When the community around a game reaches a critical mass, the very small percentage of its members who have the skills to modify things becomes significant as well.
For instance, Richard Burns Rally was not intended to be modded at all, yet the fans added new cars, new tracks, online scoreboards, etc.
In the Luanti [1] community (a voxel games engine/platform, designed to be moddable nearly from the start), one begins to see something similar as well: notable games gets mods, others don't (the former default game is a particular case; it is not exactly good but go tons of mods because of its status, and games based on it benefit from that ecosystem). Yet all use the same engine (perhaps Roblox is similar in that respect, I'm not sure if they have "reified" whole games like Luanti did).