William Niskanen, on CATO Unbound says:
Among the much larger number of members of the constitutional convention, however, there was only one vote against the provision that U.S. Senators would be selected by the state legislatures. The primary effect of this provision on fiscal responsibility is that it contributed to limiting the unauthorized expansion of federal powers. In 1913, the year in which the 17th Amendment was ratified, total federal outlays were 1.8% of GNP, almost all of which was for the military and the deferred costs of prior wars, and the total federal debt was 3.0% of GNP. Total federal outlays are now 20.3% of GNP, most of which is for programs for which there is no explicit constitutional authority, and the federal debt held by the public is now 36.4% of GNP.
I don’t think that the growth of federal spending can be explained simply by pointing out that Senators are now directly elected. There is nothing to convince me that legislatures would have appointed (or would continue to appoint) individuals that held the Constitution in high regard.
The only thing that I think we can say for certain is that “unfunded mandates” would never have become an issue. A Senator that is appointed by a legislature isn’t likely to twist that legislatures arm by tying federal funding to a coercive law. What we would probably get instead is a black check from the federal government to the respective states, so at the end of the day we have the same large check – but without the strings attached.