[ewheeler]

I'd suggest commercial aircraft as an even better analogy than cars.

Most of the ongoing costs you mention for cars still apply--but there are also the occasional (possibly dramatic) changes to the interior 'cabin product' like new seats and entertainment systems, new fabrics/branding, new business class seats/pods, changes in seat layouts, etc in order to remain competitive in their market segment. Cars rarely have such significant refreshes, but software products often have analogous design and UX overhauls that are also intended to try to keep the software competitive in its market segment. And again airlines don't need to engage the specific airframe manufacturer like Boeing or Airbus for these, but they do need some semblance of a tech team that have certain domain expertise in aircraft engineering constraints.

Airframes also have major overhauls called MROs (Maintenance, Repair, and Overhaul) about every 6-10 years, which again does not require the original manufacturer but does require significant engineering expertise. To me this is akin to certain ongoing software maintenance activities like updating a codebase to use newer library versions, major database version updates, API or SDK version compatibility, etc.