Actually, the 68K code generator of recent gcc versions *is* rubbish.
It's a clear demonstration of the open source problem: Instead of keeping the brilliantly working and well tested code generator of the 2.95 release active and try to get it working with a compatibility layer, the whole compiler was turned upside down. Certainly on good purpose, as to support more recent architectures, but that broke the entire lower layer which was, actually, no longer properly tested and checked.
Yes, the new code might be all polished, nicer, easier to maintain, supports C++ in its latest version, and C99 ... but just doesn't work brilliantly on old platforms, sorry.
So you've heavily used GCC 6.2 on 68k to write C++14 code?
And even if what you say is true, why is this an "open source problem"? Does the latest MSVC++ compiler spit out 8086 real mode code that is as good as a 20yr older version? If not, why? " Closed source problem "? Nope, just technology moving on and getting left behind.