That said I'm pretty happy with Dropbox and not going to switch. I'd love Smart Sync, though, but not at 2x the price I'm paying for Plus right now. $99 a year is great, $199 not so much.
What makes you think it's going to be surprise-discontinued?
Right now Microsoft seems really high on cloud, particularly as a means to make Office a subscription product (which helps with legacy support, as in the long run, there are no legacy versions to support in this model). From that respect, I don't think OneDrive is at any risk of ceasing to be a product, but I do think they might yo-yo the storage quotas and things like that. Didn't they do that with SkyDrive a while back?
I'd certainly rate their risk lower than Google, which does have a history of abandoning things that people really like.
Dropbox and Apple are lowest risk for me, but for two different reasons. For Dropbox, it is their product, and they seem to make money. For Apple, it's baked into their user experience, and they tend to focus on that - rather than cut costs and compete on price, they'll make it more expensive if they need to, but it'll probably continue to work well.
Edit: For me, after Apple actually decided to integrate iCloud into macOS, I felt more secure about it staying, but before that it definitely felt like a pet project that might be abandoned. I still don't fully trust it with my documents, but since I have an iPhone, I have well over 15GB of photos and videos uploaded that I'd like to see stay for a while. :)
That said, it probably won't happen overnight and it's not that difficult to migrate from OneDrive to Google Drive or such. Lock-in is low.
Groove Music is an even more recent example. Yes, the service is going away, and certain parts of the deprecation might be annoying (you have to download all purchased music before the end of the year, for instance) but all accounts can be migrated to Spotify, a previous competitor of the service. It's not like we left people completely stranded without any options.
