Discontinuing the app and having it removed from the app store are two different things, generally speaking, for Google Drive sync.
Although your concern makes sense regarding Diarium’s future, it’s a thought shared by users of any cloud-dependent app.
Given how Timo developed this app and how it’s configured to handle data, this concern is almost non-existent for me, and I hope this explanation can bring you some comfort.
Even if Timo decides to discontinue development, Diarium uses Google Drive sync. Apps utilizing Google Drive sync typically store data locally on the device, meaning that it’s readily accessible and synchronized with the user’s Google Drive account.
Other platforms like Evernote and Journey (specifically the new Journey Cloud Sync), employ proprietary cloud sync solutions...
This means that oftentimes, these apps store data primarily on their own servers. As a result, data isn’t stored locally by default, not in the same way, and may only be accessed on-demand, rather than being automatically downloaded and stored locally.
My statement may be slightly inaccurate about Evernote, but the general idea still holds truth to them. I still have an app on my phone that uses Google Drive sync but the app has not been maintained in years and I can still see all of my data.
An issue would only arise if the actual app is pulled from the store and you get a new phone, rather than still being available for download even if it is not being maintained.
Again, there would be no way to tell for sure, but I surely trust Timo more than any other developer I have bought apps from. I don’t invest in or use apps based on features, but based on the developer. Reputation matters. Communication matters. Timo FTW. I don’t foresee any issues.