This really happened a couple of months ago, and really sucks. There aren't that many alternatives to Dropbox if you require both Windows, Mac and Linux client support. I ended up settling on Nextcloud (ownCloud fork) hosted by Hetzner[1].
So far it's pretty good. Photo sync works from Phone works perfectly, and the desktop apps are decent enough. I did get bitten by a recursive folder-sync bug [2], which is certainly not of Dropbox quality but at least pull-requests are welcome.