The vast majority of companies with large debt loads don't go into bankruptcy: they prosper, they pay off debt, they thrive. But some companies do go bankrupt.
My concern regarding Six Flags and the debt that it carries is that they may, in the future, to address that burden, cut costs by closing parks, selling off parks, closing rides in operating parks, or otherwise negatively affecting my experience at one of their parks.
In all probability, that will never happen-- Six Flags will continue to be one of the country's leading park operators. But it is a valid concern, particularly, but not exclusively, for those who own stock in the company.