No, it doesn't end with the ad buy. There's the next ad buy. And "follow-up" with a client does not measure effectiveness. Certainly not at the local level with station reps talking to area merchants. Some guy's brother-in-law says "I heard you on the radio" and the guy thinks his ad's effective.
In today's world, even small businesses have the computer tools to analyze sales, inventory and expenses. If the hardware store advertises a special on Black & Decker tools, and they see an increase in sales they know the advertising worked; if the margin on the sales is enough to cover the cost of the ads and still leave a profit, they know the advertising was effective.
We used to say that in small markets and towns, the cash register was the ultimate ratings device. The car dealer could always tell the number of "ups" pre- and post-campaign and evaluate using our station. Today the metrics are much more precise for the local direct account.
And agencies and their clients carefully monitor product inventories, reorders, and such and know how a campaign is working. They also monitor share of voice and competitive campaigns, and look at all of these factors together.
You want to measure the effectiveness of radio advertising? Just count all top consumer marketers who don't use radio - especially those who were big users of radio a generation ago. All that's left, especially in right-wing talk, are bottom-feeders.
You are generalizing again. If you look at the "Rush stations" you will see that the established, heritage brands that have competitive signals are often in the top 5 billers in a market. The issue affecting traditional talk stations... conservative or mainstream... is that the demos are aging and Gen-X listeners are not using talk formats whether they are on the left, right or in the center.
As to clients, we see over time that there are shifts in the key categories. 30 years ago, an insurance ad was seldom heard and, of course, cellular phone ads were unknown. Today they are among the 3 or 4 biggest revenue categories. There is no "bottom feeder" characteristic such as you describe... just shifts in revenue categories.