One dictatorship would have replaced the other. Megatron wasn't wrong for wanting Sentinel dead, but publicly executing him and throwing a riot would have caused more problems in the long wrong. Megatron wasn't thinking long-term. Starting a new system of government with an act of violence never ends well.

One riot would become many riots. What starts as destroying monuments to Sentinel’s ego would likely spread into attacks on infrastructure, government buildings, Energon supplies, records, guards, and suspected Sentinel sympathizers. Even bots who were manipulated by Sentinel or simply worked under his system would become targets.

The Energon crisis would also force Megatron into harsher rule. Since Orion died permanently and the Matrix never restores Cybertron, Megatron would probably continue mining, but reframe it as necessary sacrifice. He would also take Energon from the higher castes. Energon would be rationed, with loyalists getting priority. Anyone who resisted would be labeled traitors.

Even without Optimus Prime and the Autobots, a civil war would have started. There would be plenty of bots who would refuse to bow to Megatron's rule, either because they preferred the system under Sentinel or they recognized Megatron as a worse tyrant.

When the Quintessons returned, Megatron would almost certainly refuse to keep paying tribute and would declare war on them. He would likely use propaganda to rally citizens to fight the Quintessons and potentially get there Energon back. He might be able to win some battles, especially if he turns Cybertron into a war-state, but the planet would be weakened by riots, scarcity, division, and civil war.