It needs at least a stable set of users, but maintaining a set of users is essentially managing the set of people onboarding and the set of people migrating off.
If you're shrinking then a competitor is providing better options, or your problem space has shifted.
Software as big as Open Office may not need to technically grow, but still needs a lot of work to stay current.
Various dependencies, like XML parsers, JPEG decoders, HTTP libraries, etc, change over time, drop deprecated APIs and need the main application to adapt. It'll also bump into the tech changing around it -- UTF8, year 2038, 64 bit CPUs, etc.
For a project this size it takes work just to stand still and keep it comfortably buildable on a modern Linux distro.
If you're shrinking then a competitor is providing better options, or your problem space has shifted.