This might be the key paragraph in this useful Times description of the increasing gridlock in the Senate:
Critics of the idea, who exist in both parties, say such a change would do great damage, causing Washington to career from one set of policies to another, depending on which party held power.
Your point being? The first problem here is that making the Senate function like a proper legislative body wouldn’t, itself, cause radical shifts of policy to result from elections. The United States would still retain a system with an unusually high number of veto points, and the power of the Senate would be constrained by the House, the executive branch, and the courts (who are most likely to challenge the federal government in periods where there has been a major partisan shift.)
And perhaps more importantly, even in parliamentary systems you generally don’t see radical shifts resulting from changes in government. Responsible party government has a moderating effect because it decreases the chances to shift responsibility for unpopular policies. And you can see this even in the American system during times of unified party government. Note, as I’ve pointed out before, that it wasn’t the filibuster that stopped the Bush administration from privatizing Social Security. Filibusters did allow Democrats to stop a few terrible judges, but they certainly haven’t been net winners here, and every time a more dysfunctional equilibrium is established this helps reactionary interests.
Anyway, it is true that policies would shift more after elections were won and lost without the filibuster (even if this shift will be much less severe than many expect.) So what? Parties that win elections should be able to govern and properly staff executive and judicial positions.