You're forgetting anti-trust. Standard Oil controlled 90% of the oil production in the U.S. until it was splintered into numerous companies. We're not really at the point where this needs to be done for tech.

Also, dynastic wealth itself doesn't sustain itself well, and dwindles in a generation. See: the aforementioned Rockefeller, whose grandchildren are nowhere near as rich. Notwithstanding that many of the wealthy (in particular, Bill Gates) are donating the majority of their wealth when they die.