While it is of course possible for 64 bit hardware to run both 32 and 64 bit code. The converse is not true. I am sure that Apple will want to exploit every possible advantage that 64 bit can possibly give them. Maintaining the status quo with support for all devices I would imagine is a hindrance in this regard. I am not privy to developer resources for IOS 7, but I do not think it is beyond the pale to imagine Apple rolling out new features that are only supported by the 5s.
Apple also wants to support the largest # of users as well, they want those users to use the latest iOS releases. Dropping support for old hardware reduces the number of folks who can buy the newer apps and Apple doesn't get their 30% cut.
If Apple wanted to support the most recent hardware, they should've stopped supporting iPhone 4 with iOS 7, which is barely usable there.
Apple releasing new iOS features just for newer models is normal, it's started a long time ago. However, it does not end the iOS support for the device, they simply just don't activate on unsupported iPhones.
Siri in iPhone 4S as an example, it required a specific hardware in iPhone 4S and was released in iOS 5. And guess what, iOS 7 still supports iPhone 4S.
The same thing will happen with iPhone 5C and iPhone 5S, it's going to get supported by the next 2-3 major iOS releases for the next 2-3 years.
Also, if Apple drops 32-bit support, they'll kill off:
iPhone 4, iPhone 4S, iPhone 5, iPhone 5C, iPod Touch 5th gen, iPad 3, iPad 4 and AppleTV. All of which are still a huge market share at the moment. It's not going to happen as a fact, so relax, iPhone 5C will be supported in the next 2-3 years.