Mobility among castes helped strengthen the Hindu community in ancient times. The fact is that after Manu Smriti became widely accepted the Hindu community began a long degradation and adoption of ridiculous superstitions. Before the laws of Manu came about the caste system allowed free movement of people to change castes according to their skill, physique and knowledge. . that time castes acted more as guilds rather than any religious social status. The laws of Manu locked in people into a single caste and this took away the rise of many prominent and brilliant personalities to the forefront of Indian society.
For example, Chandragupta Maurya was the founder of the Maurya Dynasty. But the interesting fact is that he comes from a lower caste. At his time the caste systems were never that much entrenched and anyone who had the skill and will could rise to great heights. Even the lowest peasant could become king in their own right so long as the individual has the skill and dedication. When Chandragupta Maurya established his kingdom and later an empire he was automatically promoted to the higher caste of Kshatriya(warrior caste). This was same for everyone else who followed his footsteps. Some of his descendants didn't find much attraction in being a warrior and they became priests, sages and even scholars. Even Emperor Ashoka was born to a lower caste woman but he was the one that the court of Maurya chose to support to be the emperor of India.
After Manu Smriti came into existence movement of people between castes became prohibited by law. It took away one of the core aspects of Hindu religion which is freedom. This single document later twisted the caste system into a discriminatory system that shattered the bonds of the Hindu culture.
This is your interpretation of 'Hinduism,' but the idea of a caste system that is strong and an important fixture of a society is a noted conception in other Indo-European societies. They held castes that at times were extremely rigid without the laws of Manu. Scythian society for instance, possessed a rigid and hierarchical caste system that did not permit the levels of freedom that you mention. So unless, you feel that the caste system or Varna of the lands of the Hindu is an entity unconnected to the castes of the people of Central Asia and Iran, then I feel you would have to at least admit that it is your own opinion regarding what is permissible according to the systems of varna or jaati.
The Hindu world also, I do not believe possessed a true level of social mobility in its ancient period. The idea that one can rise high is only one edge of the sabre as they say. A society that had a more social mobile mode of existence, was that of the Akkadian society and culture of the Bronze and Iron Age. Wherein, a man could become a god or a king and his royal house could become a chattel slave, for 'the Great Gods favor strength.' In the older texts from the Vedic era, it speaks of a person of any caste of social standing being free fundamentally, for the sun touches all occupations and ranks of society and grants them freedom in that sense. Furthermore, in a mentality of the world that is ever in flux yet always a constant cycle, it would make sense that one has true freedom, but otherwise is locked into a caste for this particular life for the sake of social harmony.
The case of Chandragupta was different. While legends around him differ, he was adopted by a member from the Brahmin caste, a high ranking person undoubted; above all other occupations and ranks. This is why Chandragupta was permitted to study in Taxila and learn the secrets of Dharma and become acquainted with matters not usually known; because he was 'chosen.' In other words, the myth around him do not affirm that a lowly shepherd can become a king, but that it is still the Gods who choose a king, it is the priestly caste which asserts a ruler and makes him resound across the land. Chandragupta was chosen by the Gods and appointed by the highest caste, he did not rise up the ranks and attain something freely.
Permitting others to rise to prominence may seem positive, but it also carries with it issues. Armies from China for instance worked under total conscription and a form of meritocracy. However, these practices did not produce superior armies to the foes that they often faced, who existed in terms of hierarchies, noble forces and castes. Likewise, as I often point to; the standing meritocratic army of the Sassanids was inferior objectively to the noble warrior caste armies risen by the Great Houses. For one, in a society wherein items such as bravery and skill with weapons is still useful, having a caste of humans whose sole objective is to train for war, prepare for war and otherwise does no other work, is an enormous advantage.
It may not permit some sort of genius to rise up, but that is in my opinion an overrated quality in the grand scheme of history. More important is the attempt to assure stability and longevity, which is the truly rare quantity in human states, not genius. Chandragupta's Mauryan empire was an ephemeral entity in the grand scheme of matters, certainly in comparison to states that did possesses deeply entrenched caste systems, like that of the Arsaco-Sassanid confederacies which ruled a more or less stable territorial border zone fro nigh 900 years; whilst the Mauryan empire in particular ruled its territories fro around 120 years before it receded to a territory smaller than the Nanda. One could argue the Magadhi empires displayed a level of longevity when discussed in overall terms. But this is different than the prior countries mentioned, in that the Magadhi varied wildly in their territorial expanse. It was anything but a stable imperial concept.