That said, is there a particular reason (other than a chunk of code not really necessary to functionality) that save game files don't work that way?
Java serialization was once popular, it was an initial design decision to use it, it was somewhat easy. We would actually like for save games to work more like you have described. It's a very large project, that is all, and we have a large backlog of small and large projects that we are working on and collectively there is not that much development labor available.
Re: performance requirements
These are the wrong things to focus on. First focus on functionality, what are you going to built? What value will it provide, how will it be used? You need a good product for anything else to matter. Then I'd encourage to stop worrying about specs. Simple and well done architectures are fast and perform well. Simple code is usually fast code. Once you have a reasonable design in place and functioning, then you can start analyzing where the performance bottlenecks are and where to go to optimize. It's a ways to get to that point, and once you are there the process is to measure, change, repeat. Humans have generally very bad intuitions for where performance problems will arise. It's also easy to try and fix all performance problems as optimization problems when they are really design problems. One can look at a block of code and wonder "how can I make this faster", and sometimes the right answer is, "how can I avoid needing to execute this in the first place?" That is why keeping things simple and working is very important, it lets you reason about what your code is doing. For example TripleA does a lot of calculations twice, why? Because there are 15 layers of call indirection separating the two which creates several thousands of lines of code in between the duplication.Bottom line
OUr time is short in just about every way imaginable. I'm pretty sure and somewhat regrettfully going to say, if you want something that interoperates with TripleA, it's potentially one of and likely all of: untractable and/or a truly huge project
We have wanted to for example to allow TripleA to run on web browsers, or phones. That project is probably 3-8 years away from being completed if it were started today.