One reason that is often cited for retiring old equipment is that "you can't get the parts".
This is actually innaccurate. You can get parts for virtually anything. You can still get parts for a model T Ford. You can get parts for steam locomotives. The more accurate assesment is "you can't get parts at a price that makes retaining the old stuff financially viable".
Train equipment doesn't have a strict shelf like as food does, saying, after this date it's broke and can't be fixed. It's more like the costs of keeping it running increase from year to year and there comes a point that its just cheaper to bite the bulet and get new.
Equipment is mainly retired for one of two reasons
1) new equipment makes more sense economically to maintain and operate
2) or, demands and use case have changed to the point that the old equipment is no longer appropriate. For example, it can't go fast enough, it doesn't meet safety or accessibility requirements, it doesn't meet modern comfort requirements and can't be easily retrofitted.
When will the Acela sets meet either point 1 or point 2? That is what will ultimately decide when they get retired.