That Theseus voluntarily lessened the royal power, it is not necessary to believe.
In the heroic age a warlike race had sprung up, whom no Grecian monarch appears to have attempted to govern arbitrarily in peace, though they yielded implicitly to his authority in war.

Himself on a newly-won and uncertain throne, it was the necessity as well as the policy of Theseus to conciliate the most powerful of his subjects.

It may also be conceded, that he more strictly defined the distinctions between the nobles and the remaining classes, whether yeomen or husbandmen, mechanics or strangers; and it is recorded that the honours and the business of legislation were the province of the eupatrids.

It is possible that the people might be occasionally convened--but it is clear that they had little, if any, share in the government of the state.

But the mere establishment and confirmation of a powerful aristocracy, and the mere collection of the population into a capital, were sufficient to prepare the way for far more democratic institutions than Theseus himself contemplated or designed.
For centuries afterward an oligarchy ruled in Athens; but, free itself, that oligarchy preserved in its monopoly the principles of liberty, expanding in their influence with the progress of society.
The democracy of Athens was not an ancient, yet not a sudden, constitution.