I've been using Dropbox for years but this change means I need to re-organise, so may as well look at other options at the same time. I really enjoy the ease of right-click to share, if any other service has replicated that.
The only thing I still do use Dropbox for is the share-with-whoever functionality, too. I'd be curious to hear about alternatives for that as well.
(To me it seems pretty orthogonal to syncing folders across machines, though, and I'm fine with and probably prefer using different tools for that. Which I guess I am doing, since I use Syncthing for syncing and Dropbox for sharing.)
Where i need longer duration i just use gdrive
For example I use it to synchronize the photos from my phone to my Laptop, whenever they can reach it other either by being on the same network or by connecting to the global syncthing "introducer" system, they start syncing.
Having a "cloud" system becomes useful if you need to share a file with someone who's not online with you at the same time, which is a pretty big use-case I supposed. But that is a problem that any alternative that is software only will need to solve via some service.
Nextcloud for example or also Seafile. Both have nice clients, but require some sort of server, whereas with syncthing the cloud part is optional.
Resilio Sync is decentralized as well. And lets you setup encrypted folders, so only peers with the password can decrypt them, but other peers can take part in the syncing. So I can run an instance on a VPS and not worry too much if it gets compromised. I find its Linux support lacking and wasn't as reliable as Syncthing for me. I'm going to try it again sometime.
Seafile. Centralized server, can do encrypted folders. I'm going to look at this one again, but found the sync took a real long time, and I don't think it synced file mode bits.
Nextcloud. Centralized server. Can do encrypted folders. Seemed OK, but didn't do mode bits.
Currently I'm using Syncthing, where my central always on machine is my home server which would be running anyways. I like the idea of having a 'central' server in the cloud, but need that to be secured in a way that Syncthing can't do. So I'm not sure its my permanent solution yet.
I also use Unison for much larger folders, like my code directories.
But Syncthing doesn't let you share a link to a file. For that there might be OwnCloud-based solutions.