Without fear of opposition becoming dominant, the ruling powers that be are not incentivised to placate the masses, and instead consolidate power and turn to self-enrichment. Similar to what is seen in autocratic or single-party states, eventually leading to some sort of collapse or revolution when the people reach a breaking point.
Capitalists currently control the world, and they're not going to cede any of that control voluntarily unless they have something to be afraid of.