I've never used Overleaf, but for several years I used ShareLaTeX as my primary LaTeX editor. I've since switched to using LaTeX through emacs but I still regularly use ShareLaTeX's great documentation and if I didn't carrying my Linux laptop around everywhere, I'd probably still be using ShareLaTeX. Hopefully this new partnership won't ruin it.
Eh, I've used both and have to say that overleafs interface is way better... So no, it won't ruin it.
I hope this partnership means we'll soon have the ShareLaTeX frontend on top of the Overleaf backend. That would be a truly powerful combination.