The thing to remember is that your tire is not existing in some isolated world... it is surrounded by the nitrogen/oxygen mixture. If you stop to think about it, most tires do not wear out from the inside. And, if the 21% oxygen mixture were that corrosive to tire rubber, you would still have the problem on the outside of the tire, which is the part that is touching the road, and far more important to operating your vehicle safely.
The thing about the oxygen leakage thing I have never been able to figure out... if over some period of time, the oxygen leaks out of my tires faster than the nitrogen, and I keep refilling them with a mixture that has nearly 5 times as much nitrogen as oxygen, wouldn't my tires eventually become nearly completely filled with nitrogen?
I used that last one on a car salesman one time, and he responded with "Well, I'm not a chemistry teacher, so..." I let him know that I
was a chemistry (and physics) teacher, and that caused the conversation to move along to non-chemical ways he could remove money from my pocket and put it into his
.