There is already rail connecting Hollywood to downtown that doesn't see particularly high usage. I am not sure why this specific real estate needs to be reserved for a second Hollywood to downtown subway line that as far as I am aware has not made any progress in the decade or two since it was initially proposed.
The Red Line, which you are referring to, is one of the most used subway lines in the US. Combined with the Purple Line to Koreatown, the two are #9 of the list of most-used rail systems in the US...and that doesn't include the light rail systems (i.e., above-ground rails) of the Expo, Gold, and Blue lines.
Los Angeles is the 2nd biggest city in the US. Being number 9 on the list of most used railways in the US isn't exactly a huge accomplishment when much smaller cities like Boston appear above them on the list.
