Your method would eliminate the storylines behind a #8 seeded Villanova in 1985, or a 2014 #7 UCONN, or a #6 Kansas winning a National title in 1988. The beauty of the current format is "Yes Virginia, there is a Santa Claus", and every single team in the nation has a chance to take a grab at the golden ring. Even those of us deluded Loyalty fans saw a path this year to a natty (those were some THICK orange glasses!)
A playoff system such as you propose is already used by the NBA, and it is BORING. Outside of the fans of teams involved, no one tunes into game 2 of the Eastern Conference finals. Tens of millions tune into opening weened of March Madness (one reason the NCAA takes in over $1,000,000,000 for this two weeks alone.
I'll take a hard pass, but thank you for asking.