There are many possibilities, as it is not a pre-packaged solution.
For example, if you have Synology or QNAP, there are packages to sync with cloud providers, like Backblaze B2, Amazon S3, Azure Cloud Storage, etc. So if one of your syncthing sync instances (which you can make with few clicks) is such a device, with few more clicks you can choose your cloud platform and backup to that.
Or you can be using something entirely different. That's the beauty, you can use whatever fits your needs and budget, you don't have to fit yourself to limitations of one or two pre-packaged solutions. You can do something entirely different, e.g. if someone from your family also has some NAS, you can backup to each others devices and not rely on third parties at all. The possibilities are limitless.
Organizations already have such an admin, and workers can sync whilst they're at work. Not very difficult to set up, and saves tons of costs (although a NAS is a small initial investment to make it doesn't require much maintenance).
Individuals don't, but most individuals have no clue about the repercussions of hosting their plaintext data on Dropbox in the USA. Its a ticking timebomb.
For every human being on the planet needs their own privacy. Its a common theme in the EU, but it isn't only important within the EU.
Therefore, there are two viable options:
1) You store your data locally (encrypted if you want to protect against burglars).
2) You store your data remotely, but encrypt it before you send it and decrypt it after you receive it (public key cryptography).
There is no other, viable, long-term option. Dropbox's solution is a short term solution.
Now, the question is whether you really need to sync to your LAN right away. You most likely don't. You can just sync in the evening and at night while your device(s) recharge. If you really need to sync ad hoc you have the option to punch holes in your firewall, or use a VPN. One opportunity for innovation here is to allow a user to define what must be synced right away over WAN and what shouldn't. Another opportunity is to make cloud backups easier. But these require the above requirement #2, and as you might know, public key cryptography just doesn't seem to be user-friendly.