Don't stay for obligation so ask yourself if you honestly still believe - it sounds like you don't.
If you don't still believe, pull off the band aid and tell the founders you plan to leave. However, tell them you'll stay with them through the new year to 1) buy you time and 2) not burn bridges with them.
If you don't still believe, pull off the band aid and tell the founders you plan to leave. However, tell them you'll stay with them through the new year to 1) buy you time and 2) not burn bridges with them.