A 32-bit operating system has the ability to run 32 bit applications and has backwards compatibility for 16 bit applications.
Today, a modern generation Windows 64bit system can run 32 bit applications with backwards support for the 32bit applications. However, it doesn't go back any further, so 16 bit apps are out.
So, in the event that Microsoft makes Windows 8 or 9 64bit only...that simply means there will be no 32bit version of the operating system itself. This does NOT mean that only 64bit software will work with it.
I'm sure that 32 bit programs will continue to run for at least 10 more years.