He pretty much had two options: retirement or renewal with iHeart/Premiere.
There were really only two other takers if he were to move on, but Cumulus/WestwoodOne is widely believed to be too cheap to shell out even a reduced asking price for Limbaugh, and Salem, although it's shown willingness to spend, probably would cause a lot of harm to Limbaugh's ratings simply because its stations aren't very well known.
I guess it came down to how much Limbaugh still wanted to work in radio; apparently it was enough to take whatever iHeart was offering. I wonder if the deal gives him increased time off.