2/3 of a majority to changes is not democracy; 50%+1 is democracy!

Decisions made by 50%+1 are also subject to being overturned or repealed when the political winds turn just a few degrees. The Founders put those 2/3 and 3/4 majority provisions into the Constitution to insulate the amendment process from the vagaries of politics and public opinion. If an amendment has enough support to survive the process, it's probably not going to be repealed. It's only happened once.

If 50%+1 were the standard for amending the Constitution, the rights of those in the minority would be trampled. The only reason an amendment declaring marriage to be legally defined as between a man and a woman, thus making same sex marriage illegal, has not been brought before Congress is that it probably wouldn't survive the 2/3 and 3/4 majority requirements. If 50%+1 were the rule, it would pass in a heartbeat and there'd be no legal gay marriage anywhere in the US.