One part of maintainability is cost. And a simpler mechanical engine without proprietary ECUs is going to be cheaper to maintain, provided parts are available.

If someone encounters issues with an ECU and it needs replacement at $1k-2k they might just consider the costs and that being a down payment on a new vehicle vs. repairing. Labor costs more than parts for complicated electrical/computer/engine problems. Electrical issues in modern vehicles don't appear to be easy to troubleshoot, sometimes require proprietary tools. A simpler mechanical engine could be DIY repaired and running, check out the "low-buck garage" youtube channel and the $2 Jeep series as an example.

I'm not advocating something like going back to computer-less, inefficient, stinky, loud cars, just pointing out that when we add computers to things, it makes them less maintainable to the average person.