As time goes on, I more and more have the feeling that maintenance is 95% of the value in any software system.

A well-designed system at launch is not sufficient to provide sustained value, but competent ongoing maintenance is. Phrased another way, you can sustain value by maintaining a badly designed system but not by not maintaining a well-designed system.

Ideally systems are well designed and well maintained but we don’t always have that luxury.

Circling back to your point, it’s ironic that maintenance is the lowest-value activity in terms of big tech company reward systems.