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Doing feature development is usually what triggered and started the Open Source project. It is only natural that the share of your time that is spent on feature development decreases over time in software projects as other activities grow and become dominant.
In many ways, success and user growth work as a brake for feature development.
My experience says that feature development is the primary "item" in an Open Source project that companies are most willing to step up and pay for directly. All other maintainer duties are much harder to extract money from companies for.