The accomplishment of Limbaugh that isn't popularly appreciated is how he saved AM radio from economic extinction.

For 90-odd years, AM radio has had a two-tiered hierarchy: there were the "clear channel" stations, which could have 50,000 watts, huge antenna farms and very large coverage areas (especially at night). Then there are the smaller stations, which have three classes of operation - some have to shut down at night, some have to reduce power at night, etc.

Here's a list of frequencies and clear-channel status:

https://www.fcc.gov/media/radio/am-clear-regional-local-channels

The net:net of the issue is that by the late 80's, most "local" radio stations had moved off AM and onto lower-power FM, and the small, regional/local AM stations were dying on the vine. They couldn't charge squat for their ad spots, they had shrinking audiences, etc. There's only so much radio talent to go around, and there are only so many sports fans and even farmers can listen to market reports for only so many hours a day. I remember a local AM station in upstate NY when I was up there for college, and they had a two-hour farm show from 5 to 7 AM, and then they'd break for national news, state news, commentary on same (which took from 0700 to 0800), and then they'd play 50's music for the rest of the morning from 8 to 12, national news, state news, Paul Harvey, and then country music in the afternoon. They were trying to cater to every person in town, just at different times of the day. They had a modest antenna, 1,000 watts of power, and they were dying.

Then came Limbaugh. Suddenly, stations could buy content that hooked an audience - and this national radio show knew how to market itself to local AM radio stations, leaving room for their local ads in the middle of the national show.

Following on the heels of Limbaugh, Art Bell comes up with an idea for a way out-there show in the wee hours of the morning, and it also takes off. Bell's show used Limbaugh's model of how to recruit, interact with, and reward the local stations, and suddenly Bell's program is on 600 AM radio stations as well. 

Thanks to these nationally syndicated shows, these peanut-whistle local AM stations had an audience, they could charge real money for ad slots, and they were brought back from the edge of extinction.