You vastly underestimate both how different these old systems are, how many resources it takes to support every additional platform, that the burden increases non-linearly, and what kind of weird release-blocking bugs those extra platforms may introduce, delaying fixes and hogging resources.

At minimum, slow systems require gracefully degrading GUI, which increases complexity by an order of magnitude. The graphics card capabilities are a much better first approximation than the word width.

If it took just a few extra engineers, or a few thousand, to fix the bugs that they have, Apple would have done it. The Mythical Man-Month, and all that.