As is often the case with scientific research which many people believe to be pointless, technological innovations aren’t always made by achieving the end goal, but through the technologies developed to reach that goal.
Development on COSMIC Edit has lead towards improvements to the cosmic-text library, which is used by many GUI libraries in the Rust ecosystem now. Similarly, the UX designs for the text editor improves the COSMIC interface guidelines, and puts design theories to practice. Likewise, widgets that are necessary for the editor are added to the COSMIC platform toolkit, and existing widgets and features are improved to improve the development experience for applications like this.
No one would want to build applications for a platform that lacks widgets capable of properly displaying, formatting, and editing text. Many would also find it debilitating to have a desktop environment without a text editor preinstalled. Imagine if GNOME didn’t have Gedit, and KDE didn’t have Kate.
Besides, this is a default text editor for a desktop environment. It is really not that complex. The goal is not to develop an IDE, but a text editor that anyone would feel comfortable using as their default editor on the COSMIC platform.