Tell me about it. I've been plucking away at an online business since 2004. It makes just barely enough money that I'm not quite willing to shut it down but not enough to get excited about or quit my regular job. Hate it.
My first product worked, but very average. My second project worked excellently, there was quasi-immediate traction. That second thing was resolving an actual business issue.
It’s stagnating now. It’s probably because, even though I don’t feel it, I’m bored (it’s been 7 years). I have difficulty waking up in the morning.
You need to innovate. If not just for your brain and sanity, the market didn’t recognize your first product. Evaluate whether you should trust the sign that the market gives you.
Launch a second product.
More nuanced answer: I have a project that I've been plugging away at for years now. It may never pay for itself if I value my time at the same rate that I charge it out to others, but in that time it's improved the quality and speed of my other outputs significantly.
In that regard it's probably already paid for itself, but I wouldn't sell it because it continues to be valuable to me both in that regard and as a creative outlet.
If it were a throw stuff at the wall and see what sticks saas I'd have no problem selling it for a reasonable valuation of future earnings.
If I was at that point, probably.
The thousands of man hours are a sunk cost. If you wouldn't start the business today knowing what you know now, you should probably sell.
Sunk time isn’t always invaluable.