The reason for something to exist is not to be used. He was paid while doing it, and that pay stopped, and he kept doing it. Now he wishes to stop.

The reason for something to exist is someone finds joy doing it. Especially when they are unpaid.

The sadness should be focused on his inability to support himself with a tool that clearly a lot of companies, and people are using and gaining value for.

The reason for a tool to exist is to be used, even if it's just by a singular person, other projects that aren't tools do definitely fit into the criteria "just for the joy of it" but a tool, by definition, has at least one usage, and building a tool gives someone joy from the tool being useful.

The sadness doesn't need to be focused anywhere, you can feel sad for more than one thing at a time. People can be sad that a tool they think is great, have relied on, and has been important for their use case is going away while also be sad that such a great tool doesn't get enough support from companies. Both can be true, no need to control what people can or should feel.