If Brightline reaches Tampa and Jacksonville, intra-Florida Amtrak ridership (which is large) will fall off to near-zero. NEC-Miami and NEC-Tampa service would also drop off significantly. I expect NEC-Orlando traffic would survive, but given that ridership in Georgia/South Carolina/North Carolina is anemic, the financial performance of the Silver Service will crater.
If Amtrak had any sense, it would be upgrading the Lake Shore Limited, for which there is no competition coming any time soon. But Amtrak's current CEO is an idiot.
Perhaps eventually, but i don't see this being a problem for a while (years). Right now changing to Brightline at WPB makes no sense except for the novelty. Even when the extension is complete, you'd have to get from downtown Orlando to the airport to ride - might as well stay in your seat/room, at least on the Meteor.
I agree that providing a flagship New York-Chicago service makes sense. Maybe Ed Ellis should stick a couple of cars on there...
(duck & cover)