CanCon is essentially what killed CKLW 800 as a significant hit making powerhouse in the U.S., along with other rules that torpedoed it broadcasting to a non-Canadian audience.
From everything I have heard from people who worked there and competed against CKLW during its Top-40 days, that is largely a misconception. CKLW helped break plenty of Canadian artists into the US market, including Gordon Lightfoot and Anne Murray. Granted, CKLW may have had an easier time competing in Detroit despite Canadian content regulations due to Detroit having earlier flight to the suburbs than most large cities, and CKLW was the only AM Top-40 station that effectively covered the entire metro area 24/7. WCAR covered enough it could probably have been successful, but its ownership didn't want to put the money into the programming and promotions sides. WJR could cover the entire metro but didn't air Top-40, at least not around the clock.
What really killed CKLW as a Top-40 station was FM finally catching on and reaching critical mass. Detroit had several FM Top-40 stations when no other AM on the US side was doing the format, and they ultimately forced CKLW out of the format. Efforts to save the Top-40 format by moving it to 93.9 failed, and CKLW gave up the effort in the mid-80's.