A huge number of people use 3GS these days and can't be on iOS 7 even if they wanted to, and if you throw in tons of people who got the original iPad when it was released and still have and use it, you have that much larger of a userbase all who can't use iOS 7. Apple not any developers aren't exactly worried about that causing some sort of fragmentation or other issues. If people were allowed to downgrade that would just add on to that already fairly large population of users without creating a black hole or anything else problematic happening.
Give me a evidence to support this huge number, where is it ?
Same place your talking from is where. It is one thing to have devices that are no longer supported due to hardware limitations, It is quite another to allow people to start choosing what firmware to run. It becomes a nightmare for developers and it does allow people to take advantage of exploits an update would of prevented.
You say Apple doesnt care again show evidence... Again you have none. The lack of understanding you show is only exceeded by the stupidity of the idea. Have we not learnt from Android what mess it causes to have multiple OS versions in the wild and you cant support them all causing limitations and issues.
Apple want people on the latest OS that is optimised for the latest hardware, has the latest features and the latest security upgrades. This makes it easier for developers and helps protects users.
If you like people being on multiple OS versions, getting no support and causing a security nightmare and nightmare for developers then go to Android already.