I'd argue since the beginning not just 20 years.

without id, gaming as we know it wouldn't exist today. FPS games, competitive shooters, hell even Valve itself likely wouldn't exist without id software. goes even beyond that. you could even go as deep as the modding community and all the games that have come from that. the majority of modders got their start with id games. Many rockstar game devs got their start at id or via an id product.