Good question.

If the one party is founded and sustained by people who are sworn enemies of said corporate interests, there ensues an existential power struggle between the party and the corporations (foreign, domestic, or most often both), that typically ends up reaching beyond the borders of the country in question.

If the one party quickly becomes captured by foreign interests, chances are the party was founded with that intention.

Apply this lens to the last 107-119 years of history, and most of it will become much clearer.

So who watches the watchers? In a way we all do. But instead of this being a mere idealistic aphorism, there are mechanisms in place to ensure it. We enculturate people to value equality and not valorize themselves above others, we minimize the potential benefits of corruption and keep the punishments consistent, we ensure that the watcher is not a lifelong position, we ensure that watchers do not become a separate class, we subject the watchers to oversight and approval of those who are watched, and we set up the processes so that they only function when people are working together.

This is so much more extensive than the asymmetric and byzantine setup that passes for "checks and balances" in liberal democracies. Is it still possible for things to go awry as a few bad actors try to bend the framework to favor themselves? Yes, absolutely. And that is a challenge to the people setting up the framework, to keep the wrong people out initially and to make it strong enough that it can keep its integrity once the founders are gone.