What he said (and thanks for writing that all out!).

As I like to joke, airlines are so innovative that they were one of the the first commercial users of computer technology way back in the 1950s (the contract establishing American Airline's partnership with IBM to develop SABRE was signed in 1957)--they just haven't been able to afford to upgrade since.

That may be an exaggeration, but the reality is that no matter how many upgrade iterations a CRS goes through, it generally needs to retain backwards-compatible functionality because there are so many other systems out there that interface with it (including the GDSes). Thus the need to keep the bifurcated reservation/ticket scheme even though we now live in a (theoretically) 100% e-ticket world. And although ARROW was developed specifically for Amtrak, it still was built for an (airline) paper ticket world--after all, I believe ARC-affiliated travel agents can still issue paper "flight coupons" (aka tickets) to this day for use on Amtrak.