People who think the Senate isn’t representative enough clearly don’t understand the purpose of the Senate to begin with.
On the contrary, most are very aware of the purpose of the Senate, just disagree with that purpose. This is a very weak argument I've seen trotted out to defend the institution, but it could easily also be used to argue in favour of, say, the Estates General or the old House of Lords.
I won't defend feudalism; but the UK is a unitary country; the US is a federation. That's why the House of Lords makes little sense; but the US senate does.
I generally believe that centralized countries should generally have unicameral legislatures and federal/decentralized countries should have powerful senates like the US one.
On that note, I wonder why Canada's Senate has so little power and is not appointed in a 1 province = 1 vote fashion.
The US is far more centralized than you seem to think. Almost every issue in the current period of time ends up getting decided by SCOTUS, and ends up affecting every state in the process.
Even with that, the US are a lot more decentralized than say, France or the UK.
The different states have different governments which are elected by the people (and not appointed by the president or Congress like in some countries), different regulations, different education systems and curriculums, different civil and criminal codes (in some states marihuana is 100% legal while in others it's 100% illegal); different income taxes and even different sales taxes!
The US are decentralized enough to justify a powerful senate appointed as 1 State = 1 vote.