I was thinking long time ago about the same question if someone would implement this. The point of syncthing is to have it distributed and peer-to-peer. Indeed for some use cases like sharing a link with a family member “your syncthing folder” with a “simple https gallery” is not hosted. Still the idea to have some “(shared) cloud sync” would be nice for many reasons. Including automatic off-site “backup”.
I like syncthing so much I run it on multiple desktop and servers where I sync between and use ZFS/BTRFS snapshots to go back in time.
Most people who consider Syncthing are aware of the “public cloud” but prefer selfhosted. But running you own VPS/storage instance is not a nice job. I’m also not sure how much animo there would be to have some managed Syncthing cloud storage.