I needed to sleep on your analogy, Arlington. Upon reflection, it fails because Süssmayr was at best a middling talent who obscured a lot of Mozart's unrealized intentions for the Requiem that have since been better fleshed out by 20th Century scholarship. If you're curious, I highly recommend Robert Levin's performing edition of the Mozart Requiem from the early 90s.
Unlike Süssmayr, Kevin Roche and John Dinkeloo were both gifted architects who'd worked closely with Saarinen for a decade. I'd suggest that any modifications that the two made to Saarinen's final valedictory works that were already in construction (at the time of his untimely death in 1961) were likely to be pragmatic necessities that didn't alter the artistry of the TWA terminal or Dulles.
I'd propose a slightly different analogy: Tibor Serly's completion of the orchestration of the closing 17 bars of Béla Bartók's 3rd Piano Concerto.
Full disclosure: my ladyfriend is a gifted musician who focuses her career on historically informed performance; we met at the Boston premiere of Levin's reconstruction of Mozart's Sinfonia Concertante, K. 297b.