Maybe, just maybe, the problem is developers refusing to actually move forward.
It’s especially galling given game devs are the group that most prominently complains about OS X being behind the times.
As for “needlessly” dropping 32bit: if your poorly written 32bit app starts up you necessarily force OS X (and the user’s machine) to pull in 32bit variants of almost the entire platform. Thanks for that. It also means Apple has to maintain 32bit versions of every framework. I can speak from experience here: maintaining a 32bit version of javascriptcore was a massive amount of work. Every new JS feature needs duplicated JIT work, and needlessly complicates the entire JSC codebase.
That aside, Apple manages to do this - and I can’t imagine your game approaches the complexity of an entire OS -so if you can’t make your software work on a 64bit system then the problem is your code, not Apple.