The margins in talk radio are very tight these days. Rush isn’t going to attack the base of the party. It would kill him financially.
Two thirds of the primary and caucus voters don’t support Trump. I’ve seen estimates that a quarter of Trump’s voters are disaffected democrats and independents. That doesn’t leave that many real Republicans supporting Trump.
You are correct about Rush fearing losing audience. I’ve heard him say more than once that he lost a big part of his audience by going after Perot. But it is a shame he doesn’t have the strength to do what is right rather than what is expedient.