First of all, the only time there is no negotiation is if you don't have a choice.
Will the company fold if you leave it?
I have personally been in this situation and even then I have negotiated because the owners were only able to put so much money and above that they would prefer to just cut their losses and dissolve the company.
> If you can leave the next day there's no much room for the employer to negotiate anything.
No, you just decided to give them your terms and allow the company to only approve or deny them. You take away their ability to negotiate with you in response to your demands but you did not take their ability to make their own decision. You did not dictate anything.
You would dictate if you had actual power over the company.
You are not really a dictator if they can refuse your offer, are you?