I've seen the throttle body screens pretty dirty and near clogged but never completely. But I would say it is possible especially after 15 years of neglect or lack of maintenance such as cleaning and environmental/weather conditions. And I don't care how good the air filter (including religious replacement) or filtering system is but crap like fine dust particles, road dust and carbon/soot from other vehicles' exhausts always manages to get through. Plus it is compounded by the carbon coming back the other way through the throttle body. I clean the throttle body, the screen and MAFs on all of my GM cars at least once a year as part of a preventive maintenance plan. Better safe than sorry.