You can think of maintenance as one sub-branch of software development, usually containing tasks that do not create immediate value for the business, but need to be done to be able to add new features. It's also a good way to make it clear to non-programmers that a certain amount of time needs to be spent on tasks that seemingly do not create direct value. Maintenance is a concept that everyone understands and is a good way to explain it.

Regarding the project vs product model, it fully depends on the business. Venture outside of the SaaS/FAANG/etc type businesses that HN seems to focus on and you'll see countless businesses that are very happy to just have a project done for X cost in Y time. After that, the project goes onto a maintenance contract, where the development team is just expected to keep the lights on, fix bugs and keep it secure. From my experience, it's a huge part of software development, considering the amount of systems that exist in this form is only increasing.