iCloud is great for Mac/iOS users. I never felt it worked well on Windows. It never felt right. So yes, Apple still has work to do if they want wider iCloud use. Maybe they don't want to?
But Microsoft and Google are creeping up. Their file sync is almost there with Dropbox.
OneDrive works on Mac and Windows, supports Files on Demand (similar to Dropbox Infinite - essentially placeholder files, very useful for small SSDs), and are working on differential sync for 2019.
The kicker is that OneDrive costs $99 as part of Office 365 Home, and gives you + 5 family members each 1TB storage, GBs of Outlook storage, the desktop Office apps for all devices including Android (they run really well on Android-enabled Chromebooks). And access to phone-based support. What's more, many people can get Office 365 discounted to $70 via the new Home Use Program, or even free if they're students or teachers[1].
Google One (one.google.com) is also a pretty formidable offering where available. File storage via Google Drive, a great photo storage service, tons of Gmail space, and access to Google's support teams.
So yes -- file storage is becoming a feature and the Google & Microsoft offerings will end up tempting many customers. Dropbox still does sync really well and has the best app, in my opinion -- including the only one that works on Linux, but I'm not sure that'll be a massive concern for the average customer.
[1] https://www.microsoft.com/en-us/education/products/office