Just trying to be honest, sorry it's negative. Assuming I understand the way your product works, it's just JavaScript sending a key to a 3rd party server to check if the domain name is valid for that key. I think most programmers could write that themselves into their own project in a matter of minutes. Yes, the front end is nice and would take some effort, but your target consumers are programmers and don't necessarily need a nice front end to get stuff like that done. Additionally, it's easily defeated, and in your FAQ you're actually recommending that they do additional programming to disguise your code into theirs. So I honestly think the effort of using your service is more than it would take someone to write it themselves.
It looks like you wrote this project mainly for yourself, and are just trying to market it for additional revenue. So it's hard to call it a failure altogether because it seems it's provided the necessary value to you. But I can't imagine something like this being commercially viable.
I'm not sure rigging copy protection to deny usage is a great strategy. could instead continue to grant access but notify and then use as trigger for a sales conversation. "Hey looks like you're running five instances of random webapp, we're really pleased you are finding more valuable ways to use it then we initially discussed. Looks like the contract is out of date, you're only licenced for two instances, let's get that updated..."
I'm pretty bad at this but you get the general idea.
It looks like you wrote this project mainly for yourself, and are just trying to market it for additional revenue. So it's hard to call it a failure altogether because it seems it's provided the necessary value to you. But I can't imagine something like this being commercially viable.