Don't these presenters entertain millions of people?
You may not like them, but the BBC is paying them what they are worth because they are popular. You may not like Graham Norton ET al, but millions watch his show specifically for him.
Let's say that tomorrow, the BBC decided reduce the salaries of its 15 highest paid presenter to 10% of what they are now. The stars would leave for other broadcasters, opening the door for younger, hungrier presenters.
The public might warm to these presenters, without a significant drop in viewing figures..
You might think, "Ha...everyone wins". But the economies of the entertainment industry don't work like that. Once these new batch of stars (or rather their agents) cotton onto the value they bring, their salaries would rise because that's how the free market works, and we're back to square one.
I'll reiterate this point, because I think it's key...it's the market that determines the value of a presenter. If they can deliver an audience, a broadcaster will pay.
I personally think that Jeremy Clarkson is the biggest Git this country has ever produced and I find him boorish, infantile and incredibly irritating, whenever I've stumbled across one of his programmes.
However, he guarantees an extremely healthy revenue for the BBC, and therefore, despite my opinion of his, I reckon his deserves his (massive) salary.