For a Linux user, you can already build such a system yourself quite trivially by getting an FTP account, mounting it locally with curlftpfs, and then using SVN or CVS on the mounted filesystem. [1]
What Dropbox does well though is on demand sync. Having frequently accessed files stored and synced locally and offline available while keeping most of the files off the limited harddrive space.
Other options like OneDrive don't have such capability on Linux or are not available there at all. It's very hard to find a suitable alternative especially a European one.
But, for anyone on a site literally called "Hacker News", a DIY dropbox should be trivial. The fact that anybody, especially, "nerds" or "hackers" would pay good money for what is effectively an FTP server with file versioning is insane.
I am mostly jealous that I didn't build and sell dropbox. Becoming an overnight millionaire by setting up an FTP server.
But seriously, modern approach, rclone[1] + "cloud" stoage (webdav, ssh, wasabi, whatever)
[1] https://news.ycombinator.com/item?id=8863