startups are about the work
this is why they don’t belong in start up land
running and working in a start up requires a certain type of insanity
this individual is not a fit
If the founder views you as replaceable, then why not work at a big tech which would pay you dramatically more? Successful startups are not often populated by the irrational.
Ultimately, this individual needed someone to shape and build the core of their product and the net of a series B would have been at most a wash compared to current employment.
Who raised the money that the company is using to pay salaries? When investors put money into a seed company, they're largely betting on the founders' perceived skillset and previous experience (or other bona fides like education).
One thing that most people don't realize is that being a founder means that you're inextricably tied to the company for its lifespan. Losing a founder is terrible optics and can be a death sentence for a startup. Regardless of the actual reason, every subsequent investor conversation will involve an explanation of what happened.
If you want more equity, you should ask for it! And you definitely shouldn't take a job where you'd feel under-compensated! But realistically, if you want a "founder-level" equity, you have to start your own company.
For example if they're in SF and they're hiring a senior first engineer that would maybe make 250k elsewhere, and they're offering them 125k, and they would take the classic 7% for 125k, then 7% is a good starting point. (Of course if they already have the YC investment, then that would go down dramatically)
If that equity vests over 4 years, then frankly maybe 28% is a better starting point.
But what's fair isn't really relevant. What's relevant is what the market demand and supply is. If there's some dolt who would happily take 1.5% as a first engineer ("founding engineer") for a $125k salary cut, then the founders would be idiots not to take that deal. And frankly, if that $125k salary cut gets them their dream job, then maybe they're not even dumb for doing it.
Good capitalist businesses buy at the cheapest price they can, and they owners focus on balancing competing resources (control, dividends, ownership, status, information, etcetera...). However: people run businesses and people are not rational economic actors.
A good question is: what amount of equity can you negotiate? What do you have that will convince owners to share their ownership with you?
If you are negotiating with VC, then I think the game board and the rules of the game are already rigged against founders and employees. VC sets the rules and the mileau to play the long game, and employees are lucky to get a few leftovers.
You can be a founder or join a self-funded startup, that will give you a better chance of "fair" treatment, especially if you have the skills to join people that have high integrity.
In theory if you can marginally add 10% to the business value you should be able to argue to get some amount of that. However measuring an individuals effect on a business is usually really difficult (even consultants or businesses that specialise in increasing value usually only capture a tiny percentage of the value they add).
Also different people bring different resources to a business, and anyone with a monopoly on a resource can negotiate for more shareholding. There are idealistic economic theories for how people should bid in multi-party negotiations. Note that even though multiple people may each increase the value of a business by more than 50%, that doesn't mean each should get 50% of the shares (and obviously can't if more than two want >50%).
Generally if you need to ask for shares then you have already lost the game. Either found a business and put yourself in charge, or have something the owners want and demand ownership.
Disclosure: made small amounts of money as part of a self-funded startup joining high integrity co-founders. I've had little experience of VC funded companies or employee shares. Our SaaS business was doing something we'd done before and it was started decades ago when things were "easier".