Actually, to be fair to Amtrak, I have been using their assigned seating for Acelas and NER business class, and I have had almost no problems. Well, once, I couldn't get the app to send me to the change seat page, but the seat it assigned me wasn't so bad, so I didn't really try too hard to figure it out.All of those could be supported with a modern reservations system, including car assignment by destination. As I said, it really is not a tough problem technically.
Back in the day, the manual reservation systems included size of party and members of the same party were seated together.
While I can see objections to a poor implementation by Amtrak's crack IT department, none of the issues I have seen in this thread could not be addressed by a competently designed system. As I said, the problem isn't very hard.
The only suggestion I have for them is to provide the ability to change the seat before one buys the ticket.
Basically, pre-assigned seating means that if you book at the last minute and the train is very full, you have to accept the risk of getting a less than optimal seat. But this happens with non-assigned seating, too, as I know from long experience riding the Northeast regional and getting on at Baltimore. At least you're guaranteed some sort of seat. Back when the northeast trains had unreserved seating, you could end up standing for the whole trip.
Basically, if one wants their preferred seat, it's best to either book early and/or make sure to ride the train when it's not going to be busy. In any event, Amtrak is pretty clear in stating that passengers don't have any sort of entitlement to have 2 seats to themselves. You're always going to have the chance that you'll have a seatmate if you're traveling by yourself. It's best to just accept that as part of the charm of the train riding experience. And, to be honest, despite all the horror stories I've read about unpleasant seatmates, I've never been stuck with such a person. I guess it might happen in the future, but I'll deal with it when it happens. Some of the worst "unpleasant passenger" experiences I've had have been when I've had 2 seats to myself and the optimal window view, but some idiot a few rows away ruins the ride for some reason or another. Anyway, it rarely happens, most Amtrak riders are well-behaved, and the rides are uneventful from that perspective.