I've been developing for proprietary and web platforms for a long time too. I really don't see that the rise of mobile is in any way coming at the expense of the web.

Who are these development organizations who truly enjoy maintaining five different apps for all the major desktop and mobile platforms and who are aren't going to make the jump as soon as HTML5 delivers everything they need?

Obviously there will always be applications (antivirus, encryption, etc.) for which a browser is poorly suited. But this percentage will never be bigger than it is today.