If it succeeds and becomes popular, now it's a time sink. It's no longer something you tinker on when inspiration strikes. You have obligations.
At that point, it feels like slave labor, like a thing you are required to do without pay. And you feel like if people value it, they should support it somehow.
Most folks doing open source probably aren't great at monetization. If they knew how to do this as a business model from the get go, they probably would have.
Relatively few people feel free at that point to just walk away and say "Not my problem. I created this for free. If you love it, you maintain it. If you need something reliable and up to date, then pay for a commercial product. I've given you all the time I'm going to give you for free. I'm on to new hobbies."
No, instead they get all excited that people want it, resentful that people won't pay for it, and they rightly recognize that it has value. Now they want to be compensated.
And the world has Nobel Prizes intended to encourage people to create brilliant new breakthroughs to benefit humanity without necessarily knowing how to monetize it, so we have this idea embedded in our collective subconscious that if we do a good thing, it ought to be rewarded.
I've done a lot of volunteer work over the years. I've thought a lot about such issues.
I think we would do well to develop some systems for helping people monetize their thing if it gets popular. But we could also work on the cultural side of this and help people understand that you need to think about how much you are willing to give and that if your thing gets popular, making money with it is an additional job to do, not an entitlement.
And if you are bitter that it doesn't pay, one legitimate option is to walk away or only give it however much time you actually enjoy giving it and not one minute more.