The founders never even intended that the majority be able to vote.

Really, their core quibble with the Crown, and the sticking point that prevented any peaceful solution (because compromise would have seriously threatened the political power of the nobility in Great Britain), was that they wanted rich people to have the franchise and to be able to hold high-level offices, not just aristocrats.

The problem is, when the majority rules, the result is inevitably a shitshow.

The founders understood that, which is why they set things up the way they did. Nothing has changed since then. If anything it's even more true, given the new and extensive ways the elites have to manipulate majority opinion.

There are numerous countries in the world that are literally democracies, not republics, where the majority does in fact rule.

Quite a few of them are not shitshows.

So I consider your first sentence to be proven false.

I think where the founders got it wrong is not expecting the population to be so ignorant and easily swayable. For instance, school voucher measures, which would benefit so many people, tend to get shut down due to lobbying efforts from rich and powerful teacher's unions: https://www.forbes.com/sites/jeffreydorfman/2016/11/13/teach...

If people were wiser and did their research better, these kinds of campaigns would be ineffective and the rich would lose their sway.

The founders also probably didn't expect our government to grow so large and so corrupt. The richest and most corrupt entity in the United States is the government itself. The vast majority of the government is not even beholden to people at all in any way, shape, or form. This includes the tens of thousands of employees in the federal bureaucracies, as well as organizations like teacher's unions who are funded indirectly by tax dollars. Many, if not all, monopolies today are also artificially created by government policy, allowing for an unholy union of corporations and the government.