90% of startups fail. This one is not one of the 90%. You might learn something by sticking around even in a non-senior management position since there are so few successful startups around.
Your startup is about to enter a critical phase where it goes from startup to midsized business. This is the second most critical phase of a company's growth and you might want a front row seat for this show.
As long as you hang around people that you can learn from and they are willing to teach you, as long as you are being taken care of equity-wise and salary-wise, why should you care? If the transition to midsize is successful, the most critical part of the growth in size will take place at the level of midlevel management. If I were to work in a non-senior role for a startup that turns into the next PayPal, do you think I'd be crying my eyes out that I wasn't in a senior management role? :)
In my case, I joined the evening MBA program at NYU, got my management to write a yearly letter to the IRS that I needed this education to maintain the minimum requirements to keep my job and got reimbursed 50% of my tuition during the entire time I was going from my MBA (that was the late 1980s) and essentially functioned as midlevel manager of my firm since I was responsible for all IT operations at the firm aside from being a senior environmental engineer at the firm. They did offer me a path to partnership but I didn't take it - I couldn't stomach the idea of wearing a suit. They've changed policy since but I have been long gone, too. The firm is listed in the ENR (Engineering News Record) 400, has a multistate presence and is the largest envionmental planning firm of the New York Metro area. And I was there when it went from a seven-person startup to midlevel size of 100. After surviving several recessions - and several not so smart top management decisions - it should be around 400-500 by now.