The first 64-bit Macs came out 16 years ago. The first 64-bit Intel Macs came out 13 years ago. The 32-bit to 64-bit transition on the Mac has not been an abrupt one.
The reason Microsoft killed 16 bit support (In 64 bit editions of Windows) is that very few people were actually running 16 bit apps at that point. Where Microsoft made a decision based on what their users were actually doing, Apple has a made a decision based on what it wants their users to be doing.
The amount of time isn't really relevant. If lots of your users rely on 32 bit software, you should put in the effort to make them work. Apple products are expensive, and I think it's reasonable to expect this in return.
Regular protected mode code has built in support that allows easily running 16 bit apps, so maintaining compatibility was a no-brainer. The moment the trade-off changed, Microsoft decided to drop support for 16 bit apps.
Personally, I don't mind that very much, because dosbox is a great product that bridges the gap perfectly.