Why is Wall Street concerned that other types of investments, like bonds or CDs, could begin to offer a more reasonable rate of return?

Such concerns would seem unfounded, even frivolous, if a kind of "natural balance" prevailed among the various types of investment and savings options available today. 

In other times, investors split their money among stocks, bonds, and assorted other savings and investment instruments to meet differing needs and to suit varying personal preferences.  And under more normal circumstances, if stock buyers were concerned that a reduction in unemployment might signal an era in which bonds and other investments could become incrementally more attractive, they might have calmed themselves with observations that (a) the only investors who would really be affected were those "borderline" types who weren’t sure whether they wanted to be in stocks or bonds in the first place, and (b) whatever nibbles the bond market might theoretically take from stock prices would likely be offset by a countervailing tendency for share prices to increase, as a reflection of rising profits and dividends in a healthy, growing economy.

What's the implicit message, then, when today’s stock market gets the jitters in response to news of fuller employment?  

Bear in mind that if this nervousness were unwarranted, somebody would be making an immense fortune buying up shares that had become temporarily undervalued.  Word of this success would then in short order be all over the financial press, and the market as a whole would be less jumpy the next time around.

But since this scenario has not actually occurred, we seem to be left with only one conclusion:  that today’s investors in the stock market really don’t have the kind of options and freedom that others have had in the past, and that many of them don’t truly want to be in stocks in the first place. 

This would mean that what’s been fueling the Dow’s ongoing series of record-breaking highs is essentially just captive money, put up by investors who have no place else to turn.