So why did Amtrak invest in the Acela in the first place? The more and more this thread goes on the more I am starting to think that the Acela (aside from its service) is a dying horse.
I don't think anyone ever suggested that Acela was going to be a TGV, or at least no one that knows anything about anything. Acela basically serves the purpose for which it was designed quite well. It has played a significant role in moving more people to rails along the NEC. One can always argue about whether such could have been achieved in some other way. And yes, until significant additional track segments become 150mph capable, mostly south of NYP, the current stretches appear to be mostly for the purposes of bragging rights.
In addition to Jishnu's excellent response, I believe that there were a few reasons behind Amtrak's purchase of Acela.
First, the Metroliner service was loosing market shares to the airplanes, especially as the fleet aged. Acela was a way of getting bright shiny new cars and a sleek looking trainset that implied speed, even if it didn't really deliver all that much more speed than the Metroliner. It also provided some of the amenities that business travelers demand, things like tables, brighter interiors, and guaranteed electrical outlets at every seat. In this regard it does appear to have worked, and quite well as a matter of fact. The last number that I saw showed Amtrak with a greater market share than it's ever had, both in the NY-DC market and the NY-BOS market. I don't believe that it's done much for the BOS-DC market though, and I don't think that anyone ever expected it to do so.
Second, Acela provided Amtrak with a way to increase its fleet size, both in terms of electric locomotives and in terms of cars. Out of Acela grew the HHP-8 engine, and of course Amtrak doesn't need to assign any AEM-7's (or E60's back when they were still in service) to haul the Acela trainset. Out of the Metroliner coaches, we got more Amfleet I's to increase train lengths and even provide more runs in the mid-west. We also got the very nice Club-Dinette cars out of this, as those seats were the Metroliner First Class seats. Again, this is another success of Acela, IMHO.
Third, it provided the reason and means to electrify the corridor from New Haven to Boston. This is an overwhelming success for everyone. Passengers no longer need to sit and watch the clock tick away in New Haven while crews swap engines. This alone probably chopped 15 to 20 minutes off the running times of every through train between Boston and NY. Additionally it saves thousands, if not a few million dollars every year since Amtrak no longer has to pay crews to swap all those engines and to keep all those engines (diesel & electric) on standby in New Haven. Finally it makes Metro North very happy, as Amtrak isn't tying up MN's platforms.
Finally, Amtrak and in particular the Amtrak President at that time George Warrington, was betting that if everyone saw the nice flashy new train that could, they would then start throwing money at Amtrak to not only fix many of the deficiencies on the NEC, but also to build new high-speed systems elsewhere in the country. This unfortunately was a dismal failure. Not only did it not encourage more funding for Amtrak, many critics of Amtrak seized upon it as the argument/reason not to give Amtrak any more money at all. They pointed out that Amtrak spent X millions of dollars on a train that at that time could only hit its top speed of 150 MPH for a total of 18 miles along its 450+ mile run between Boston and DC.