You're getting some weird responses down below.
The Constitution is not an infallible document. The Founding Fathers knew this, and included specific language to change it. Some floated the idea that the Constitution would even expire and have to periodically be rewritten entirely.
And the reason Senators are not based on population is a compromise to states that had smaller populations. What we lost in that compromise was equal representation. It's states' rights at the cost of citizens' rights.
I guess it's just easier to paint this as an argument over how the Senate does work rather than how our government should work, so that's how they'd prefer to frame the argument. Otherwise, they'd have to justify why some peoples' votes are more valuable than others, instead of equal representation.