Have you been working on your engine by yourself or do you have a team?
The engine is written by myself and to be honest, it is stable for some years now, so you dont need 15 years. But nevertheless, a modern engine is very complicated (I'm talking about multicore, physics integration, shader, scripting, AI etc) and it will costs a few years to make it stable (I'm not talking about the first visual showoff after 2 weeks of work, I'm talking about the month and years of refining your engine). This is time lost to make actually a game. When I started the engine, there were really not a lot of options to use an existing engine, but nowadays there are hi-professional engines almost for free for indies out there.
If your goal is to invest only 2-3 years for a 'non-casual' PC game, then your decision comes down to either make a game using an existing engine or to make a engine with a simple demo.