I don't have hard figures, but I'm willing to bet that the vast majority of people coming from Orlando on I-4 are NOT planning on going to downtown Tampa. And vice versa.
I notice that the huge majority seem to zip right past the "Downtown" exit for both cities. Conservatively let's say at least 75% or more of the cars go right past the downtown exits of both Tampa and Orlando. It certainly looks that way from driving it. And if you ever go downtown to either city, you know this is true. What people who are there came because they work downtown and commute from suburbs, not from across the State!
In fact, a large percentage on drivers on I-4 aren't even stopping at all! They are just passing through from east coast to west coast, or the reverse.
And it's the semi trucks that cause a lot of the bottle neck on I-4. You're not getting them on the bullet train!