I think there's a culture of expecting reasonable businesses to seek unreasonable goals.
Until the valley can move from "always dream big" to "well sometimes, just plan sensibly", it will be perpetually unstable.
I remember a strip mall across from a corporate office park. Every couple months, a new .com would go in and another would go out of business, but in the strip mall the same places had been there for 20 years. The web isn't going anywhere, there's nothing wrong with providing for the long term.
It's very likely that only so many "home runs" can be hit...that doesn't mean you can't do very well for yourself by focussing on a niche need, a good product, and excellent service...
A winning business model since mankind first began trading and bartering...
You make a very, very good point...
Presumably someone might think we are in a bubble and be right, but not have enough information to know when the market will correct or to what degree.