The timing of the original "All Aboard Florida" announcement leads me to suspect that the causality might be reversed. I believe that the Amtrak on the FEC planning may be what initially caused the FEC to become interested in running their own service in the first place. I don't think it's too unlikely to believe that the FEC hired a consultant to check if Amtrak's numbers were realistic and got a response like, "Actually we think they're underestimating them."Amtrak has looked at service on the FEC. I'm not entirely sure why it didn't come together, however, but at least to begin with Amtrak never had the right to bulldoze their way onto FEC since FEC got out of the passenger business prior to A-Day. This is similar to the lack of service to Des Moines: The Rock Island never joined Amtrak, so Amtrak never had "automatic" access rights there.
The reason I believe this to be the case is due to how quickly the FEC moved forward after the Florida HSR project was canceled. They clearly must've been doing studies even before then, and the 'go/no-go' decision must've been made prior to the announcement. Thus, I don't think it's very plausible for them to come to a decision so quickly if they hadn't been looking into the issue prior to the cancellation of the HSR, though I'm certain that played into it, the timeline definitely suggests they were already looking into the idea when that happened.