How do you know Siemens would take that deal in a heart beat?There is a work around for that, in the airline business and some extent railroad business, one negotiates a trade in of present asset. Amtrak did it with the SD40Fs on new F40s. Siemens would take that deal in a heart beat for 5 years or more guaranteed work and bigger contract.
Manufacturers will only take a trade-in if they think they can resell it somewhere else. Who else is looking for several dozen new locomotives and/or passenger cars (they still will have to/want to build the cars from the current order) other than the very places theyre buying them back from?
Theres virtually no way theyd be able to make a deal to buy them back, unless it was at a price that means Siemens would lose money on the total deal.
The more DMUs you run together, the less cost benefit there is to running DMUs. One big issue is that DMUs are regulated as locomotives by the FRA and come with stricter inspection requirements and more frequent mandated shop visits than a plain old passenger trailer car. This means that maintenance costs are higher on a DMU than a standard car, and spare ratios have to be higher.Then run two DMUs together. Before that though it allows Amtrak to have better yield management, just as Amtrak does with the frequently sold out Acela. You are right, DMUs would not be right for all circumstances but having one or fewer types of equipment yields lower costs. Again out of the airline playbook.
That quickly cancels out any savings from cheaper (but not really cheaper) DMUs and/or fleet commonality (a single-level coach can be common to electric and diesel corridors, but the same cannot be said of an EMU vs. DMU, unless you go with dual-mode, but all that does is make it even more expensive than it already is).