It seems to me you're looking at this from a technically proficient developer's perspective.
It's easy (trivial) for a technically proficient person to setup any trivial cloud storage system they like on just about any provider's platform they like.
It's not so easy for anybody else (read: the vast majority of other users). Dropbox's value proposition isn't commodity storage on somebody else's servers: it's the UI/UX that backs it, and the particular implementation details.
Dropbox isn't installed by default on any platform.
Apple, Microsoft and Goole all offer 'cloud' storage services (they aren't a product, stop calling it that) and all have a user-facing platform on devices: macOS/iOS, the various Windows, Android/ChromeOS.
Dropbox is effectively a feature. That isn't to say they can't remain in the market and possibly even become profitable, but just making a sync client whose biggest recent news is the skeezy install process on macOS isn't going to cut it.
It's effectively the only cross-platform solution that works very well on all 3 major desktop platforms and both major mobile platforms. It seems like no one else has the incentive to take other platforms seriously. Especially, for anyone who collaborates or shares files with many small groups or clients. Even though it's not first-party it is the lowest-friction option, because you can't count on all your collaborators being on the same ecosystem.
Over several years, my experience of trying to use Google Drive or OneDrive on the Mac has been very frustrating, with significantly more CPU and RAM usage. It's ridiculous that OneDrive still doesn't do differential sync. AFAIK, neither has official clients for Linux.
If the market treats them like a product, they are a product. Also the number of people outside of techie communities that care about either Apple computers or "skeezy" installs on them is tiny. Mac has a tiny, tiny fraction of the computing market and the vast majority of MacBook users are about as tech savvy as the vast majority of windows laptop users: that is they aren't savvy at all.
A product is something you buy once. A service is something that is provided to you over time to use, usually in exchange for money. My issue was people calling cloud based storage a 'product'. It isn't, it's a service.
You seem to be somehow ignoring the little glass-faced elephant in the room: iPhones/iPads. The number of people who use these devices is not tiny.
The people who don't care about Apple but are still potential Dropbox customers are still using an OS: either Windows, Android or ChromeOS. The manufacturers of those OS' also compete with Dropbox. So what's your point about Apple exactly?
Non-technical users are exactly my point. Why would grandma use Dropbox, when its almost certain the device has built-in cloud-syncing storage from the OS manufacturer?
Exactly. What happened to Google Maps market share on iOS when Apple make Apple Maps the default option (even considering that at that point GMaps was way better than Apple's alternative)?
It's easy (trivial) for a technically proficient person to setup any trivial cloud storage system they like on just about any provider's platform they like.
It's not so easy for anybody else (read: the vast majority of other users). Dropbox's value proposition isn't commodity storage on somebody else's servers: it's the UI/UX that backs it, and the particular implementation details.