Morgan Stanley and E*Trade Financial Corp. were Wall Street opposites: One an investment banking firm with a historic pedigree that catered to corporations and the wealthy, the other a discount brokerage that had pioneered cheap online trading. Now they’re merging, for reasons that say a lot about how the U.S. financial landscape is being reshaped. After a decade in which Wall Street giants mostly stood pat, in part because of the fallout from the “too big to fail” bailouts of 2008, the desire t