You need conviction in your own beliefs, and the willingness to ignore others naysaying.
You don't get to make their decisions but you do get to choose the decision makers.
The startups that do well are the ones that are working all the time.
When lack of structure fails, it fails all at once. What works totally fine from 0-20 employees, is disastrous at 30.
By the way, that's my number one piece of advice if you're going to join a startup: pick a rocket ship.
It takes years and years, usually a decade, to create a great startup.