I could go and say that the aerospace engineer is also just doing "plumbing". The engineer is not going to design an engine for instance. "All" he does is specify what kind of engine he would use and then plug it to the fuel system and the wing structure. The engine was designed by someone else at a different company most likely. (Example: the Airbus A320neo uses PW1000G turbofan engines made by Pratt & Whitney. "neo" stands for "New Engine Option" by the way). This happens with a lot of the parts of an aircraft. Dismissing that as "plumbing" would be absolutely insulting to the engineer.

The same is true for a lot of software development. It is true that I am not going to design and develop ALL of the components of my software. For some things I will use libs and frameworks someone else made. I don't see a reason to be dismissive about that.

For instance I will be using the libs our hardware manufacturer provides to talk to the avionics bus of the A320. I don't see a reason to redesign our own ARINC429 avionics bus libs, but I also don't see a reason why this would make what I do not "actual software development".

A better question: Which would you rather have doing the inspections an your aircraft?

As I know as couple of people who either build aircraft for a living or build aircraft for their personal use, neither of them is an aerospace engineer. But I'd happily fly in their aircraft.

Irrespective of the title, can they design and build the object in question. This applies to all fields.

The point that we must look at is whether or not the problem before us can be solved by us. It doesn't matter if you build from the ground up or use some pre-built parts. Is the solution going to work and solve the problem at hand. If it does then you have success, if it doesn't then you have failure.

It is a matter of understanding the problem (in terms of the problem space), what solutions you use only matters if you can't solve the problem.