OS 9 -> OS X was supported by the Classic runtime for years.
PPC to Intel was supported by Rosetta for years.
I suppose if the announced that this was the plan, and gave people years to migrate across, it might be something they'd do. But it would kill basically any technical use of the platform (development, scientific computing). I find it much more likely they'll evolve iOS "up" to support users for whom a closed appliance approach is helpful, whilst adding options to OS X to cater for those who want a more traditional computer but need a bit more help.
But they didn't - they made Rosetta and supported it a couple of years just to give the developers and the users time to update their apps/computers to Intel versions.
Remember how much business they lost last time they did this (the intel switch, or even the os x switch)? Oh wait...