This isn't some side project like Wave that they'll cut off. It sounds like a pretty core business. If anything it will get renamed and take on a different form in a couple yrs, but I doubt they'd ever stop offering a cloud service like Drive.

But still, this is a very valid point. It's hard to trust any Google service and it's risky to make any significant investment of time structuring your life around them. This is why I don't use Google Music anymore. I invest far too much time into my playlists and music collections to trust them.