"Financialization" is the key.

The upper class is the investor class. They live off of passive income generated by their owned capital- interest, dividends, rents. Their wealth grows faster than they can spend it.

The lower class is the working class is the debtor class. They live paycheck to paycheck, exchanging 80 hours of labor for another two weeks of housing and food. Any big thing they 'own' is bought with debt, and interest on those debts eats up most of what they would otherwise be saving.

So obviously it's a fuzzy continuum but the middle of the middle class is someone who has both investments and debts and is making nearly the same amount in passive income as they are for paid labor. They are living better than most anyone without investments, but still can't stop working tomorrow. They hope to be able to stop working someday.

Those people are motivated to defend the system as it is because the same financialized economy that keeps the rich getting richer is- or was- also their only ladder out of a lifetime of wage slavery. They may even know that a radical socialist restructuring of society would be better for them, but have no faith in the moderate liberal political parties to pull it off, and aware of how precarious their position is.

The ultra wealthy could lose half their wealth and half their income, and barely even notice. The almost-wealthy would fall off those ladders and never get back up. But when it finally comes time to eat the rich, they'll be the closest thing to an actual rich person the hungry masses have ever even seen.