I think we're looking at this the wrong way.

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.