When you have no clients (and you’ve been pushing the product long enough) is easy to realize that you better drop the product and go back to the drawing board.
When you have many many clients, it’s a no brainer to go on (and become filthy rich 🙂 )
The problem is when you have some clients (e.g. imagine that you make, let’s say, 1500 USD per month with your product). Not enough to make a living (at least in many countries/cities, specially if you have to support your family) with the product but enough to be willing to stick to it.
Sure, you could try to take yourself out of the equation and automate as much as possible the product and all the tasks surrounding it (support, publicity,…) but still it will still occupy some of your time (or it will slowly die so you’ll lose the 1500 anyway).
You could also stick for a while and see if you can bring up considerably the revenue but how long should you try? When it’s better to just stop and try a different idea that could work better (or not, and then you’d lose the 1500 again!).
An alternative option would be to try to sell the product before moving on the next idea (not sure how much you can get for a product that makes 1500 USD per month, probably no more than 20-30.000 USD)