Mr. Train Guy, passenger convenience and service attractiveness is clearly now secondary with Amtrak to operational efficiency.
Reiterating points I previously made at this topic, to serve the Airport facility first means that rent would need be paid to the airport authority, which I presume is avoided by staying away from there.
Operationally, an arriving train could not be expected to be at the Maintenance Facility for at least an hour after its airport arrival. That depends upon when the FEC has traffic to handle to or from its yard at Hialeah, as all that traffic X's the "diamond" at Iris. A train to make an on time departure from the airport would need leave Amtrak's Hialeah yard one hour prior than its current "set" time. Somehow, I think the term " late set" would become far more endemic at MIA than it presently is.
At present with the "one a day" frequency, that would mean four additional movements over the X-ing. If frequency is restored to "two a day", that four becomes eight.
Handling Tri-Rail over that FEC X-ing is a separate agreement between those two parties.
The only Amtrak passengers that would be benefitted by the airport station would be those who desire to rent an auto (and willing to pay airport rates). Amtrak must have knowledge of "how many", but I would think that number is low. Passengers will either be "met" by friends/relatives, or will take a taxicab to destination. I guess I should acknowledge that those desiring to use a jitney, such as Shuttle America, would be benefitted by the airport as well.
All told, this is simply reiterating why I think Amtrak is best advised to "stay put".