What this stock price says to me is that investors are betting that this story, even if it's correct, is not the last word, and that Microsoft (or someone) will eventually get them.
If Yahoo! does reject the offer, their stock price should, were the market completely rational, fall a lot. Back to what it was before Microsoft made the offer in the first place.
Yahoo's the same company it was two weeks ago; the stock was just worth more since MS was offering a premium.
I wouldn't count on it. Look how many of 1908's giants are still big today. A few, but most are dead and gone. This book talks about some of the reasons why:
OK. I doubt that Microsoft and Yahoo will be around in 2108. It seems inconceivable that Google could fall though, especially if Google.org's energy research pays off.
Unfortunately it's not free. The stats cited in the book are that only 20% of the top 100 from 1915 were still there in 1995. Think about the technology landscape 100 years ago, and how much it's changed. I think there's every chance that Google will either not be around, or be radically smaller or different than it is now. It could be even larger, too. It's simply not possible to know. What is evident though, is that with time, companies grow and die.
Bonus fact - this is one of the oldest companies in the world: