This is a similar situation to the headphone jack, Apple pushing change, but this shouldn't be a surprise to anybody. Apple announced this a while back, the issue is the sudden update by the op.
Microsoft supports DOS apps back to 1984 in Windows, they are forever stuck with this and Intel is forever stuck with producing processors that support 16 and 32 bit apps.
However, I can only guess that processors for mobile phone without 32 bit instructions, can use the precious circuitry space for other things (what other things, I don't know) Could be smaller die, better power savings, etc.
But to be fair, app developers have been given ample notice.