One difference is that Dropbox isn't the only option: there's Google Drive, Amazon Drive, OneDrive, Box, OwnCloud, Syncthing, Resilio Sync, SFTP, and many other "plain dumb file sync" tools to choose from. That certainly increases the security footprint some. You might know I synchronize a KeePass file with a cloud provider, but you might not know which cloud provider, and while the chances of any one particular cloud provider being hacked are somewhat large it seems, the chances of all of them being hacked enough at the same time that you find the KeePass file needle in my particular cloud footprint haystack is hopefully pretty rare. It's also something that if you get wind of an attempt in progress you can mitigate/defend by switching sync services or removing it from sync altogether until the attempt ends...

Because it's "dumb file sync" with a number of options, there are also some really interesting options with interesting security footprint trade-offs of their own. Resilio Sync, for instance, originally known as "BitTorrent Sync", supports peer-to-peer sync and more interestingly supports "encrypted peers" where you can have a cloud provider participate that "knows nothing" about what is inside the synced folder but can still share/sync it with your devices that do.

Similarly, if someone develops something crazy like a killer secure and somehow user friendly IPFS sync option tomorrow, you could switch immediately.

Not that similar. The reward-to-effort ratio for attacking LastPass is much better, you'll gain access to the passwords of so many users. With Dropbox, its less likely that you'll find something valuable and therefore less worthwhile to try. Most users would store their vacation photos, perhaps income tax info which might be useful but not as instantly exploitable as passwords.

Not really. If dropbox is compromised and they get your .kdbx file they've still got to be able to break that (unless you use the same password for dropbox and your kdbx file....which is not a good idea).

There's self-hosted team solutions around, they just don't get as much exposure.

(Like mine: https://pave.software/ )