You don't actually need to sue people to make GPL have an effect. Some companies might casually infringe copyright, but they are less likely to do so once informed about it by the public. At that point, it becomes willingly infringement. Willingly infringement on a commercial scale is one of those key legal terms that turn fines into millions, and in some countries, can even become a criminal offence rather than civil (criminal offences are handled by the state rather than by the complaint).