Apple drops old hardware when they have a good reason to do so. They decided 64-bit x86 was the future so they relatively quickly dropped PowerPC and 32-bit x86

Apple drops old hardware when they have a good reason to do so. They decided 64-bit x86 was the future so they relatively quickly dropped PowerPC and 32-bit x86. Since then, they haven't been dropping old Macs at all. Their approach to hardware support on iOS has been similar: dropping devices with too little RAM or really wimpy CPUs/GPUs but otherwise extending it as far back as possible (some say too far back given the performance on these older devices).