Once more: I'm not talking about your OS, I'm talking about your hardware. YOUR DEVICE is the old thing. Once a device is no longer supported (again hardware not software) expecting any freemium app to still waste time developing for it is a fool's errand. I could pull out my first smart phone from nearly 10 years ago and put the newest OS on there, that wouldn't suddenly increase the processing power of that device. It still has the technical limitations from when it was originally manufactured. And honestly that's a bar that probably wouldn't be reached because parts degrade.
A lot of those crashes you've described are ones that happen when the game needs to do some more intense calculations. It wouldn't surprise me that there is a fail safe in either your device, OS, or the app itself that force closes when the processing surpasses the limits.