It's true that developing apps within someone else's ecosystem can aid addressing the chicken-and-egg issues through discovery mechanisms offered through that ecosystem.
But that comes with riders too, Total & absolute dependency on the platform being the primary.
- Cannot establish our brand as our product would be always under the primary ecosystem brand (e.g. iMessage or Messenger).
- Monetization possibilities are scare or even impossible( Messenger ToS prohibits digital asset sales to avoid conflict with Playstore/Appstore ToS).
- Stability of the platform affects product directly(Days when Messenger API goes down, Our product goes down too).
- Ownership of the data is complex, We could be liable to protect the data of our customers but we don't have total control over that data.
I've been affected by several of the above issues because I've built Messenger bots in the past and had conducted ~ 100 Million conversations through them.