I disagree. OneDrive works where there is OneDrive app because there is no open api that could be used to develop the app. Same for rest of the apps. As for Office - MS made it's format so convoluted that we had monopoly to use it and only recently more and more people use open formats (especially public offices providing forms to print in PDF, sic!). There is one thing using open standard / providing an open API and creating own implementations that offer smooth experience and there is another creating locked-down "ecosystem" where it only works when you use apps from the MS... even exchange alternative implementations are somewhat "hacky" at best.
I guess you haven't use Linux in a while?
Besides with all that money gained by being monopoly in the '90s and early '00s MS had enough to spend to polish the experience. Contrary to Linux, which is based mostly on os contributions (talking about user-facing stuff, not the server-focused one that is quite often driven by for-profit Canonical or RedHat... but they do still publish it as opensource so everyone benefits)
OneDrive, Dropbox, and Box all work fine with their own apps and with apps from other companies. They do this because all work with the Windows API for file access and add themselves to the Explorer and File windows in a standardized way. Overall, OneDrive does more and better integrates than the others, but it's not exclusive to OneDrive. For example, Dropbox has deposit folders (share a link and anyone can drop files into it, but no one except the recipient can see the contents), which OneDrive does not. Dropbox also supports LAN-based replication, where OneDrive requires a manual copy for that or forces users to replicate via the cloud -- a real problem for low or metered bandwidth users.
For the Office formats, you're conflating various time periods. DOC documents were convoluted and proprietary, and possibly and the very first version of DOCX was too, but current DOCX is open and it has been for well over a decade. As file formats go, I still prefer WordPerfect's for its stream orientation (e.g., if bolding some letters or words, there's a character at which the bold code starts, and another at which it ends, nice and simple, easy to find and remove or change). Word's object-oriented design sounds good, but it's a pain for actually working in compared with WordPerfect's stream approach: hard to find formatting, even with Word's Formatting panel, and even harder to adjust without breaking something -- try fixing a broken numbered list in Word, ouch, but at least it supports complex lists, OpenOffice/LibreOffice/Google Docs don't even support these more complex features, especially Google Docs, which is more like Notepad than it is like Word .
MS WAS (past tense) rough to do business with. Now, I worry that it's bending so far over in the other direction to be friendly to its competitors that it's hurting its customers. We customers win when companies fight hard for our business, not when they sideline their competitive natures to collaborate. There's a fine line between working together to establish standards that can help customers and collusion that hurts customers. I also think Nadella is so cloud and service focused that he doesn't believe UI or end user UX matters. I don't know how else to explain the kind of painful-for-users mistakes that MS continues to make under his leadership (while obviously doing amazingly well in other areas, as indicated by the MS cloud and AI growth and their stock performance).