P.S., not sure how I glanced by this point before:
Hinduism (Shiva as supreme being within Shaivism) is the world's oldest religion and Boedha are the most still intact.
Not intact in the least.
Many if not most of the modern sects of the Hindus are almost unrecognizable compared even to the diverse syncretisms of the Hindu Golden Age, let alone the religion of Vedic times.
Your allegation that Shaivism be the worlds oldest religion, and by implication that it was the original Hindu orthodoxy, rather prove my point that nothing is intact about any of these religions.
Way back when, the only Hindu supreme being was the impersonal Brahma (neuter noun), whence sprang the
Trimūrti (a trinity): Brahman (masculine noun; the Creator), Vishnu (the Preserver), and Siva (the Destroyer). In the beginning was only Brahma; all existence may be said to be no more than an illusion imagined by Brahma; and all things shall return to Brahma, only to re-emerge in an eternal cycle. The closest Western concepts to Brahma are in various aspects such things as Chaos, whence sprang the universe, and Fate, which controls the destinies even of the gods.
The syncretisms and variations that developed as Hindu orthodoxies over the centuries and millennia form an astounding tangle. They all share the same Vedic roots, of coursesomewhat in a manner
analogous to how Sanskrit shares linguistic roots with Greek. That is what distinguishes them from such outrageous heresies as Buddhism, or the skeptical philosophy of
Lokāyaka (a highly intelligent rational atheism which rejects all mysticism, denies the existence of all gods, and holds that human consciousness is a material bodily process that ceases at the death of the individual), etc.
The same processes operate on all religions. Judaism was originally polytheistic (as seen at Elephantine). The Christian denominations of the Fifteenth Century would all have been condemned as heresies by each of the many different Christian sects of the Second or Third Centuryand
vice versa. Zoroastrianism later had Mithraism as a direct descendant, so to speak. The Greek religious beliefs of later Hellenistic times were substantively different from the beliefs of Homeric heroes (although here, the difference seems less significant due to the implicitly pluralistic nature of Greek polytheism). Etc., etc.
The bottom line is that men create gods in their own images; and as societies change, so do their concepts of divinity.