Adding other sensors isn't free. Every minute you spend on developing techniques to process inputs from other sensors, not to mention integrating their conclusions with that of other sensors, is time, money, and energy you could have used to improve your optical system.
I'm not saying I necessarily agree (though I find his position intuitively compelling), but he clearly thinks that it's easier, faster, and cheaper to bring an optical-only system to a point of reliability than it is to bring a mixed-sensor system to the same point.
And sure, there might be a better way to get down a ski slope, but skis would be a pretty good starting point. And they guarantee you don't end up in an impossible situation because you're doing things a fundamentally different way than the system expects.