But for 1000 users or more, you will need someone to support the software. And probably only that person knows the intricacies of the system. It obviously costs the company some money to devote a person to this open source product. Now if that person leaves, it is going to cost [in time and money] to find a replacement.
Often you don't need a whole person and you can split the role with several people even then to spread the risk. Even with SaaS or paid support issues are resolved slowly and require a technical person on the customer end anyway.
Open source software is actually pretty easy to support. Windows servers, Exchange and Oracle are all pretty bad and the support isn't great either + they cost money.
