Possibly, but Valve isn't exactly a prolific developer in terms of titles and timeframes. They would not restrict a big budget title the very small marketshare this box would likely have in the first couple of years, and other publishers wouldn't be keen to jump on board when one publisher controls the strings I bet. I would also majorly piss off their Windows gamers, which is their bread and butter and will continue to be so for the foreseeable future.
Big Picture mode gives me promise, they certainly have aptitude in this area. It's not perfect but damn it's impressive for a first showing.
There is always a risk with exclusives. I agree though. The big console makers have a bigger wallet to take risks with exclusives in order to prop up their platform. Valve is a private company though, their finances are not really public. They could be sitting on billions for all we know. I doubt it though, since their Steam commissions aren't very high.