If they aren't making things work with you, they definitely aren't going to make things work without you. Are you vested? How far away is your cliff? Is your equity ridiculous?

Regardless of how important you are to your current employer, if their money dries up they are going to stop paying you and terminate your employment. Even if you are their key employee.

If they really want to keep you on, tell them you want much more equity because you are taking on a lot of risk and the startup doesn't seem to be performing strongly.