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.

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.