[vunderba]

This isn't really a very good example for two reasons

1. The concept of licensable game engines during the development of doom in the early 90s was hardly a common thing, and certainly nothing along the lines of a low barrier, low cost entry like unity used to be

2. Id software is very well known for basically being a tech company more than a game company, and later went on to make a great deal of money licensing out the doom and quake engines to other companies who could then focus on creating games without having to build a bespoke engine