Looking at what they're doing with Outlook I'd bet money that they intend to replace the desktop version of Excel with the web version sooner or later.
I agree but expect that Excel will be the very last app of the suite that they make web-only. Excel heads like my wife absolutely hate the web version.
That's got to be the death-knell for Excel. It might be better in every conceivable way but going web-only only makes Sheets look more attractive to the people doing the buying. And you can run JS in your Sheets right now.