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Haiku already has its own third-party 'native apps' that directly use its APIs and also promotes them in HaikuDepot [0]. Users from other OSes will always ask for other apps and if the OS has a 'modern browser' even when WebPositive is based on the same technology as Safari. i.e, WebKit. But they'll still ask for Firefox and Chrome.
The fragmentation argument can be made from a maintenance point of view. For typical platform support, it's easier to maintain and target one OS that has a single unified stack and SDKs (GUI toolkit, sound, input APIs, etc) than to do this on multiple distros and to only officially support 5 Linux distros, which is why Windows and Mac are always targeted easily. There is no 'standard' SDK on Linux distros, which is why one user could be using ALSA or OSS, X11, XWayland or pure Wayland with Sway, etc. Haiku has first-party default system components which it maintains.
It also sits in middle of macOS and Linux, being that its open-source like Linux and has a standard defaults and a SDK like macOS. So maintenance-wise, maintaining one of each OS like macOS or Windows sounds less expensive than targeting 3 separate distros here.
Imagine if Haiku had fully funded developers working on the OS. It would be out a lot sooner with more features, but obviously that isn't the case.
[0] https://www.haiku-os.org/get-haiku/r1beta2/release-notes/