First, Paypal doesn't just have a one time implementation cost. For example, refunds being limited to 60 days on Paypal will cause issues over time.

Even if the cost was fixed and upfront, a Paypal integration isn't an intelligent thing to prioritize for most startups. There are likely hundreds of other things a startup could do to increase its revenue by more than they would save in CC processing fees, so they should focus on those things first.

Eventually a Paypal integration will make sense, but likely not for a processing discount. Instead it will become useful for international payments, and even then most startups have enough headaches with Paypal that they choose to keep Stripe as the default checkout option, and use Paypal as a secondary option.