Attacking the problem by reducing user freedoms and increasingly monopolistic control is not the answer, even though Google's PR department would tell you otherwise.
So the question on if this effectively reduce scams is the first question to answer.
The correct way to do it would be to whitelist other good stores, and allow developer mode installs with an extra process that says explicitly I am extra sure this may be danger, but no. This would reduce Google's income streams.
The way I see it, it must be attacked the way default Internet Explorer was attacked.
More like fighting teen pregnancy by mandating chastity belts... With the same ultimate problems too: those most determined to overcome the block will make use of bolt cutters or their digital equivalent.