Quite simply, Amtrak should "just stay put" for any passenger convenience gained is more than offset by increased costs and operational disruptions.
Amtrak will have to pay another party for access to the Intermodal Center that they presently do not pay to access their own. The existing Miami station is "made for the convenience of the Operating Department", and no matter how much Marketing rants at now One Mass, saving additional expense is far more determinable than Marketing's dreams of additional Miami business.
Now let us consider disruption to FEC operations with eight additional "long train" moves over Iris X-ing. Yes, Tri-Rail goes over such many times a day, but those are three car "scoots" handling revenue passengers on each move. FEC already is making concessions to host additional passenger trains over their rails, as the connecting track through the NE quadrant of Iris is already built for the expectation that Tri-Rail will serve the AAF terminal - Miami Central.
Now let's consider passenger convenience. True, Mr. v.v. noted above that he and party will be arriving on 97 and plan to rent an auto at the IC. I could be mistaken, but most passengers are either going to be met by family or friends, or use taxicabs. For such meets, parking at the existing station is secure - and free. At the IC, secure yes, but hardly free. Taxicabs know when the trains are going to show up, and however absurdly expensive they are in Miami, will get fares.
All told, passenger convenience is questionable; the additional costs and potential to disrupt other railroad operations is quite determinable.
Amtrak, just stay put.