BMW has been producing the same engines for 20 years...their 4.4 liter engine has been in production for 15+ years ... every single year they modify the engine to perform better... they keep adding more HP and Torque to the same engine over and over again... the manufacturers purposeful De-Tune their engines so they get 15+ years from the exact engine ... Do you know anything about cars?
KN filter WILL no way shorten your engine life...that is a complete false statement.
In fact changing your air filter (which i assume you are referring too) more frequently only prolongs your engine life.

10 years ago we were suppose to change the engine oil every 3 thousand miles... now since most vehicles come from the manufacturer with a maintenance contract for 2-4 years for free they have extended the engine oil change to 8-12 thousand miles... same exact engines ... the 4.4 liter from the 90's that you were suppose to change the oil every 3 months or 3 thousand miles is being changed every 10K miles plus has about 250 more HP.

You also don't increase your emissions because emissions are controlled by the catalytic converters in your vehicle...

Added more air to the engine makes the engine burn gas more efficient thus reducing emissions and burning more clean.