Feingold's stated goal is to make the "Senate ... as responsive as possible to the will of the people". That was certainly not the vision of the founding fathers. They designed the upper house, with elections that were less direct, and less frequent, than those for the House of Representatives, to be less subject to the popular will.
This article by the Senate Historical Office quotes some of the founders:
Madison explained that the Senate would be a "necessary fence" against the "fickleness and passion" that tended to influence the attitudes of the general public and members of the House of Representatives. George Washington is said to have told Jefferson that the framers had created the Senate to "cool" House legislation just as a saucer was used to cool hot tea.
That vision of the Senate has changed over time, especially with the introduction of direct election of senators, almost a century ago. But the Senate's debate rules, that require a three-fifths majority to cut off debate, still carry an element of that "cooling the passions" concept.
Still, in this era of direct election of senators, I agree that it's anomalous to allow appointment of senators, when there is a strict requirement that House vacancies must be filled by election.
It's interesting to note, though, that, in the aftermath of the September 11 attacks, there was some support for going in the other direction, and allowing appointments to the House, under extreme circumstances. Here is a statement by Ron Paul, opposing that idea, which was not adopted.