Depending on the country that's going to be some mobile payment options, banking apps, social media or government ID. If even one of those are missing, people will pick another brand, something with the Play Store.
I just looked up WeChat and Douyin, for example, and they are working on HarmonyOS versions: [0].
0. https://www.scmp.com/tech/big-tech/article/3267411/huaweis-h...
But I think it's a mistake to assume that the Play Store is equivalent to your developer footprint for a platform.
Google has been fairly insidious about integrating OS level APIs with the Play Store, so that's blurred the line a bit here - but if the OS is otherwise "Android-like" enough that it's just a matter of choosing a different target in your build and linking to different binaries... it'll be very easy to port over the developer base that's interested.
Seems like Harmony is doing exactly that, and already has a large swath of companies that are planning to support the OS - so while some of that is likely because of government influence in this case... it's not required (ex - I'm thinking of Google and the whole lawsuit with Oracle around the Java APIs)
Huawei had a 4% market share last year. Is that going to be enough for something like MobilePay in Scandinavia to ensure that they are present on a platform, from a company and country they don't entirely trust? Without the MobilePay app, the number of people who'd buy the phones drop to almost zero. The same happens if Danes can't use their government ID on the phones, sales drops to almost zero, because the device just lost a major feature.
I don't think it matters that Huawei has a large swath of companies ready to support their OS, it has to be all of them (or at least the right ones).
They might pull it off and get every one on board, but the investment has to be really small to justify the work for such a small user base.
Countries like Iran, Russia can be next target for obvious reasons.
In SEA likes of Grab and Gojek can be financially motivated to create apps. If they get a 4-10% marketshare it will make sense for Meta to port its apps to protect its position.
YouTube will be a blocker as Google has strong reason to not support any alternative platform even if it will make sense for the App. Same happened for Win Phone earlier.