Competition itself can't protect against net neutrality violations, because even small-marketshare ISPs can break your startup's product for some customers.
Almost none of these customers will ever even learn that the failure is the ISP's fault. And even if they do, who will switch ISPs just to use some not-yet-proven product by a new startup?
Meanwhile your competitor's product will work well, either because they were too big to mess with or because they paid whatever the ISP was demanding. Maybe you should too!
This is why Europe needed net neutrality rules, and actually passed stronger rules than the U.S., even though the system you describe was prevalent there. Amusingly, before the rules existed, the increased competition in Europe had actually led to more net neutrality violations there (specifically, blocking or throttling things like VOIP and BitTorrent) because ISPs in that competitive landscape were under more pressure to cut costs!
My source on this is Stanford's Barbara van Schewick, who is a leading expert on net neutrality policy and whose work informed both the U.S. and E.U. rules.