Apple must maintain the security of its devices by updating them.
For sure older devices will work slower with newer softwares and we don't have to be even smart to know that.
Every device has life and actually Apple is a company that supports its devices much longer than any top Android maker.
You shouldn't consider an old iPad working on older OS a good choice since your security is not taken care of as it could have been if you had the latest OS.
You might say that Apple could have released some security patches for iPad 2 instead of allowing an iOS 8 upgrade but put yourself in Apple's position. They are now selling the 6th generation iPad and that is one of the ways they make money.
Imagine you are Apple and you sold iPad 2, 4 years back and still trying to make it as safe as possible after a total of 4 years besides the fact that the hardware is old and is not able to keep up but you still push it because security is the first priority.
If I was Apple I also wouldn't have released only security patches for older devices since after 4 years of service to my customers' old devices; I believe I was gonna deserve making some new money from them to buy my new updated devices...