The built-in "Check In" app is really great (it automates the "text my parent/friend/relative when I get home" flow, including texting obvious delays from GPS tracking data). iMessage apps are a good idea in general even without a "deep bench ecosystem". Though obviously when it was added Apple was hoping for something like the ecosystem seen in the Asian messenger app space where WeChat and others became the primary app stores and "everything" became a messenger app for a while. (Even in Europe there have been points where WhatsApp had a huge app ecosystem.)
Arguably part of why those app ecosystems worked so well was because they were a cross-platform app experience and iMessage only working for Apple devices and most iMessage apps only capable of strong experiences in "blue chat" iMessage user only streams was always a hiccup for why Apple didn't see a larger uptake in the iMessage app ecosystem.
Arguably part of why those app ecosystems worked so well was because they were a cross-platform app experience and iMessage only working for Apple devices and most iMessage apps only capable of strong experiences in "blue chat" iMessage user only streams was always a hiccup for why Apple didn't see a larger uptake in the iMessage app ecosystem.