Android and iPhone app stores are already saturated. The app store for HarmonyOS isn't. This could be a good opportunity for new mobile app developers.
If it’s good, and Huawei follows up on making it truly open source, I suspect every Chinese manufacturer would at least have a line of Harmony OS phones, because they could always be the next trade war target.
And once the Chinese manufacturers are supporting it, you can also add a huge chunk of the South and South East Asian markets as your customer base as well, since a lot of users in those areas basically just buy the latest Xiaomi or Huawei phones.
Chicken and egg problem.
New devs need users to buy their apps to justify developing for a new platform. The new platform needs a wealth of apps (especially common apps) to get users.