He was the founder of PRC, and was the only leader who did not go throught China's selection system to become the top leader, it was true that no one can replace him when he was alive during his time. He was indeed an exception , but China indeed needed such a strong leader back then, China had been very fractious for almost a century already, every province was a de facto independent government like today's Taiwan, China has 30 provinces, think about having 30 Taiwans.
Chinese society was filled with old social illnesses and injustice, women must bind their feet, wide spread monogamy, opium addiction, prostitution, human trafficking were just some of them, old habits die hard, China needed a strong leader to deal with those problems with a iron fist. It may seem ruthless to outsiders but he layed the foundation and prepared China for her future fast development free of those social woes. Sometime you need to completely break an old to make a new. Don't fix it if it ain't broke didn't fit in China during Mao's time.
Do you know why India can not develop fast and rival China in development? Cause you didn't have a Mao who can wipe out all the social woes which are forever holding India back.
All leader after him had to go through China's well established selection system and all served China very well.