Often times tools (in general; not specific to tool labs) are abandoned by their maintainers, such as T152049: New Maintainer needed for LanguageTool WikiCheck on Toolforge (used to be hosted outside tool labs, LGPL, defunct), and T92963: Fix tool kmlexport (tool labs tool, seemingly open source but no license, functional but unmaintained and can break from time to time). These tools are seemingly widely used prior to abandonment, but their prior maintainers could not continue the maintenance due to lack or time, energy, resource, or willingness.
On the other hand, while we have the Right to Fork Policy and the Abandoned Tool Policy, they operate on the assumption that there will be a developer showing up that wants to fork, adopt, or usurp the tool. The question remains on whether the developer would show up at all, as most users of such tools are not developers and may not have the capabilities to maintain the tool.
For bots, we usually have a bot requests page in which, whenever a bot gets defunct, someone will post a notice and a capable bot owner may take over; I haven't found similar pages for tools though. There should be some list of abandoned-by-maintainers tools so new maintainer can find and adopt them. The adoption could be done under Abandoned Tool Policy (if already on tool labs), or via forking to tool labs or other parts of labs if necessary (for FOSS non-labs tools).