Even if I can replicate your math, I am not sure all assumptions are correct.
But the main incorrect assumption is the fact that a digital signal can and is heavily compressed. A 2X compression is about what is normally expected knocking bandwidth down to 12.5 Mb/sec, or enough to allow a multicast or 2. in the total 19.4 Mb/sec OTA bandwidth.
And it may also explain why many choose to broadcast in less bandwidth intensive 720i or P.
And I certainly do not claim any expertise in this subject area, I am confident someone on this forum knows far more.