Some thoughts:
(1) Let's not forget that where Brightline is able to have all of its stations "built to spec" since they're starting from scratch, even if Amtrak had an ideal spec for the cars it isn't likely that they could make said spec work at all stations. There's still going to need to be some mix of bridge plates (where the gap ends up too large) and low-level boarding (for a lot of off-NEC stations as well as a few situations on the corridor as well) even with all of the bells and whistles that Brightline's design could theoretically throw at them.
What I think you're likely to end up with, btw, is something like "75 sets of cars that remain coupled 99% of the time and then a few spares for if a car really goes down".
(2) I have been informed that there are issues emerging with the Amfleets as time goes by. Even with a swift order, some of the Amfleets will almost definitely make it to 50 years of service (depending on how fast Siemens can churn them out and how many cars are in front of Amtrak's orders/interspersed with them). Four cars per week, 50 weeks per year would give 200 cars/year. Brightline has at least another 50-70 on order, VIA has their order (160 cars or so, IIRC, plus options), and the ex-MSBL order (137 cars) is also there. So there's probably at least five years of business lined up at those rates, and that's before any states/options, Brightline's Tampa or LA-Vegas projects, or an Amfleet II/Superliner replacement project potentially get involved, and getting to those rates probably requires some ramp-up time as well.
The thing is that, at some point, even well-kept equipment starts having issues. If this were still ATSF running the Super Chief, I could see that. Unfortunately, Amtrak has had several major periods of lousy maintenance (notably under Warrington, but I've heard there have been other spells) and I have been advised at times that some issues are lurking. Getting the fleet to 50 years is in the cards. 60 years is plausible. 70 years would be pushing it, however, and that's not even a bad statement on the equipment...that's just where you start running into something going wrong and the large-scale use of Amfleets is going to be touchy by then. That's not saying that a few states might not opt to buy the equipment and run it for a decade or two...but I suspect that it will, at a minimum, be relegated to relatively isolated use (most plausibly by states looking to start service when a new equipment order isn't feasible) by mid-century.
