I've made my living working fulltime on a single open source project for more than 15 years now.

I think it is important to differentiate between different kinds of projects that people might undertake, and 3 particular categories always come to my mind (you may have more):

* "plumbing" - all that infrastructure that isn't something you'd ever use directly, but the tools you do use wouldn't function without it. This work is generally intense during a "startup" phase, but then eases back to light-to-occasional as a stable phase is reached. It will likely happen whether there is funding or not, but may take longer and reach a different result without it.

* "well defined goal" - something that a person or a team can actually finish. It might or might not benefit from funding during its creation, but at some point, it is just done, and there's almost no reason to think about continuing work other than availability and minimal upgrades to follow other tools or platforms.

* "ever-evolving" - something that has no fixed end-goal, and will continue to evolve essentially forever. Depending on the scale of the task, this may or may not benefit from being funded so that there are people working on it full time, for a long time.

These descriptions originate in my work on software, but I think something similar can be said for lots of other human activities as well, without much modification.