Is the reason that Prophet Muhammad was the last of the messengers due to the increase in people's intelligence, such that they no longer need a messenger, knowing that each century is more misguided than the one before it, and that misguidance stems from obstinacy, and that the outcome and regret are the same whether it is done knowingly or not?

The statement that people in later eras do not need the message and prophethood because they are intelligent and can independently know guidance is incorrect. The need of people for the message is continuous until the Hour, and may even be more intense in later eras due to the complexity of life. However, if the statement means that the establishment of religion and the proof against people is not dependent on the physical presence of the Messenger himself, as long as the last message is preserved, then this is true. The death of the Prophet (peace be upon him) does not mean the disappearance of his preserved religion and his established proof, as Allah Almighty said: (And Muhammad is not but a messenger. [Other] messengers have passed on before him. So if he was to die or be killed, would you turn back on your heels? And he who turns back on his heels will never harm Allah at all; but Allah will reward the thankful.)