Flexibits' weakness is engineering, though. They could get eaten by a competitor that works its way up to a full calendar with a stronger foundation. Fantastical 3's move upmarket in response to Calendar.app improvements hasn't been easy for them so I don't know how much further they could push themselves.

That's not to say that a new competitor will put Flexibits out of business, but if the market for calendar management expands, I wouldn't expect Fantastical's share to grow proportionally. So there's an opportunity.