A few things:

The founding fathers were often in their (often early) 20s. They were not infallible, nor was the system they created.

That's why they created mechanisms for evolving that system.

Except we rarely do. "But Constitutional Amendments", people say.

Actually, one of the doctrines of the founding fathers was that the whole system should be reviewed, head to toe, every 10-20 years.

Everything is very selective. Infallible when we want it to be. "Oh they didn't mean that/like that" when we want. And completely ignore other parts as inconvenient.

> The founding fathers were often in their (often early) 20s.

The youngest delegate to the Constitutional convention was Johnathan Dayton of New Jersey, who was 26, and there were three more under 30. There were more over 60 than than under 30.

The people that are frequently cited as being "Founding Fathers" in their "early 20s" (or "between 19 or 21, because we're not sure exactly when he was born", in Hamilton's case) are people who were that age in 1776 and ended up playing an important role. But the Constitution was drafted more than a decade later -- there was a war, plus time under the first system of government under the Articles of Confederation in between.

> Actually, one of the doctrines of the founding fathers was that the whole system should be reviewed, head to toe, every 10-20 years.,

No, that was not a "doctrine of the founding fathers", it was a belief of Thomas Jefferson expressed later in a letter to, as I recall, John Adams, specifically (the upper limit of the period at which he held any law or constitution needed to expire was actually 19 years, based on actuarial data and a set of assumptions he had about what was necessary and acceptable in terms of avoiding the living being ruled over by the dead.)

You can tell it was not a widely held "doctrine of the founding fathers" (or, more to the point, of the Framers, who are the ones actually relevant to the Constitution, though the two groups have considerable overlap) because instead of expiring by its own terms in 19 years or less, the Constitution was permanent, with a very difficult method of amendment, and that method of amendment was specifically barred from changing certain parts.

There weren't really very many widely shared "doctrines" of the Founders or the Framers. They weren't a hive mind or a cult or even a group as ideologically aligned as the coalition that makes up either of the US's current major political parties. The idea of shared doctrines or a single unifying vision behind the Constitution are mythologies created after, and requiring deliberate disregard of, the facts.

They certainly were not infallible, but the question is whether they were unwavering advocates for democracy.

All of the restrictions they put in place, such as the electoral college and who was able to vote at all, along with the writings we have, suggests as a group they were not.