First of all America is a Republic not a Democracy, at least not in the pure sense of the word. In a pure Democracy 51% could vote to take the rights of the other 49%. In a Republic we accept majority rule but at the same time the majority must respect the rights of the minority.
However what you are referring to has to do with the Senate rules for fillibuster. That is not the same thing. There is no constitutional provision either for or against the fillibuster. The fillibuster is an idiotic procedure whereby one Senator talks and talks and talks about some arcane subject until everyone else falls asleep. Its a stalling tactic used to prevent real discussion until everyone gives up and moves on to another subject. It should probably be done away with.
Again - I am not only speaking of the filibuster. I'm attempting to present a larger concept than just that. However, the focus on the filibuster recently seems to be allowing for people to assume I am speaking in narrower terms.
I think I answered the broader question in the first paragraph. It was clearly the intention of the founders that our government should not be a matter of two wolves sitting down with one sheep to decide what's for dinner.
Our constitution was designed to protect the rights of the people and limit the power of the government. Your elected representatives and all judges are sworn by their oath of office to support the constitution. The founders did that to protect your rights and mine. Beyond that they also divided government power among the 3 branches and further split the legislative branch into two. I think the founders did a pretty damn good job of it but unfortunately their result was not airtight. The constitution can only stand if there are people who will stand up for it. Unfortunately too many Americans don't understand the importance of the constitution and would happily sell the whole thing down the river in exchange for more government handouts.