And if you want a sign of the old experiment, IIRC that's why you have the little digital displays above the seats on the Acela.
I do think there is an off risk of this actually losing Amtrak revenue. For the sake of simplicity, let's assume you've got a train with two seats traveling...oh, I'll just use WAS-BWI-BAL as a city set to make it nice on my brain. So you have three passengers: One traveling WAS-BWI, one BWI-BAL, and one WAS-BAL. Ideally, the WAS-BWI and BWI-BAL pax will pick one seat and the WAS-BAL pax will pick the other...but if the first two pick their seats first and don't pick the same seat, then the WAS-BAL pax is going to come along and be told "no seats available" while you have an empty seat all the way through. For the record, I've run into this with sleepers on more than one occasion.
This problem mostly doesn't come up on airlines because almost all flights these days are simple A-B flights; of the A-B-C flights I know of, many have restrictions on riding on either leg (e.g Qantas' SYD-LAX-JFK flight cannot sell tickets LAX-JFK for legal reasons). There are some exceptions (Cathay Pacific's JFK-YVR-HKG flight comes to mind, as do a few "milk runs" out there) but those are exceptions and not the rule (not to mention that most milk runs do not exactly have a load factor problem). Amtrak, on the other hand, can easily timetable a train that runs A-B-C-.....-W-X (and yes, a Boston-bound Virginia-originating Regional can easily run up to or over 24 stops, even counting BOS/BBY/RTE as a single stop) so the risk of losing more than a hatful of seats due to mid-route turnover really isn't trivial at peak times (and I don't think you want to lose part of your capacity at those times). With that being said, limiting this to the Acela ameliorates this issue somewhat (a weekday WAS-NYP Acela will, IIRC, only have between 4 and 8 intermediate stops, and NWK will be R/D-only and can effectively be consolidated with NYP for our purposes) and I'm sure that there are some workarounds in terms of dynamic pricing that could reduce the amount of this further...but I'll also admit that I'm kind of wary of a situation where Amtrak is trying to charge me an extra $10 for seat 12C in a given car versus 12C in the next car.
I think there's also a point to be made that at least as things stand, if the car is mostly empty and I got a lousy seatmate? Right now, I can move with no problem...and I see a certain value in that. To be fair, if you set up a decent back-end to allow the Conductor to make a swap on a space-available basis that wouldn't be a bad thing.
I recognize that Amtrak can probably kick out some extra revenue from charging folks to advance-select a window seat (and I've been advised that management probably has this on the brain) and it's always possible that they could offer you the ability to buy the neighboring seat at a discount (I'm thinking of things like Air New Zealand's "skycouch"), but I've got to admit that on balance I hope this falls on its face so hard that it needs reconstructive surgery...but I also lament the switch to e-tickets since that killed off what was a de facto "fully refundable" system that had survived well into the 21st century, so my desires as a customer aren't necessarily in line with what is best for Amtrak from a business perspective.