I mean, true...but I don't think the average user is paying for the service rather than they're paying for not having to worry about setting up everything needed to get syncthing working.
I don't consider myself a luddite in any way, but within five seconds of reading syncthing's install instructions even I basically just said, "yeah...no." And I say that AS a nearly 12 year semi-advanced linux user. It's not that it's difficult. But difficult enough to not be worth it for the average person.
The best part is it works with Android as well. Whenever I turn my computer on, all my photos on my phone sync to my computer to a folder that gets regularly backed up (using Vorta which is an excellent and easy to use open source backup program for Windows, Linux, and Mac)
I set it up last month. I’ve rarely experienced had such a smooth setup process. Was putting it off for years because I had assumed I would need at least several hours. Right now I have one on a server and then every device syncs to it (thought it would be easy to set up backups that way)
this was my experience too. kept putting it off because I assumed I'd need to tinker a bit. didn't at all, worked immediately with only the simplest configuration. genuinely amazing, I wish my software worked that well.
You know Dropbox? Google drive? OneDrive? That's file synchronisation. Files across multiple devices kept in sync by the software provider. Except in the named cases above, all your data is uploaded to their servers. With syncthing there's no cloud server, just your devices operating over the internet. So you have some backup responsibility to cover.
Caveat: I've never used syncthing and I wrote the above with a total of 10 seconds of reading their website and so it is entirely possible I'm completely wrong about everything and so I emplore you to do your research.