right, XML is extremely verbose - especially FlightGear's way of using XML, which is primarily due to the way the property tree works internally.
However, let's keep in mind what XML is supposed to be used for (data MARKUP), and what it is being used for in FlightGear, especially FDMs: as a poor-man's programming language.
Obviously, a real coding language is much more expressive than XML - but XML's verbosity and form make it accessible to non-coders, of which we have many ... I don't think those people would be too happy having to use C, C++, Nasal or Fortran instead of XML
JavaScript/JSON could be easily supported in Nasal, too - but we don't see many people using Nasal that way. Also, Nasal performance will be inferior to XML-based data structures parsed into STL containers.
Abstraction layers come at a price obviously. But in this particular case it's simply because JSBSim doesn't support "real" scripting, but only XML-based building blocks that need to be chained together.
Meanwhile, the FG AP/PID system (its building blocks!) has even become superior in comparison to JSBSim.
Progress is mainly about accessibility - we wouldn't be having 300+ aircraft/FDMs in FlightGear if people had to literally "code" their FDMs - not because they couldn't do it, but just because they think that coding is some highly technical skill - without realizing that coding in an ill-suited markup language dialect (aka "XML") is making things more difficult (and verbose) than necessary.
Most of the people who have developed complex FDM/AP systems obviously have demonstrated that they have the mental capacity to also work in terms of actual "code" - it's mostly syntax that differs after all, but concepts will remain similar (loops, counters, functions, inputs/outputs, state management and stages).
Funnily, complex systems like these are easily modeled in purely functional programming languages, which would be quite a bit more advanced than most/any C++ or Nasal code we have - GLSL may be a different beast due to its concurrent nature - but people claiming that they cannot be bothered to "learn coding" while creating massive FDMs and JSBSim systems are only really fooling themselves, because they've
already become coders in niche "coding language" based on XML and aerodynamics/control theory building blocks