Cut your losses and move on. You will only have so many opportunities to play the startup game in life... and probably many less than you anticipate.
They may be very likeable and connected but they are not performing at their job. They need to be driving growth by bringing in revenue, leads, funding, etc. as well as strengthening the business by reducing vulnerabilities (like having only 1 person that knows the technicalities of the product). If they are screwed after you leave that is more their fault than it is yours - that is a lack of management foresight.
Just be straightforward but nice with them - you won't be burning any bridges.
They may be very likeable and connected but they are not performing at their job. They need to be driving growth by bringing in revenue, leads, funding, etc. as well as strengthening the business by reducing vulnerabilities (like having only 1 person that knows the technicalities of the product). If they are screwed after you leave that is more their fault than it is yours - that is a lack of management foresight.
Just be straightforward but nice with them - you won't be burning any bridges.