This is also going to be easier on Paula. You know that the first question of any interview for the next year will be "What do you think of the new judge on Idol?" (Come to think of it, that was the first interview question last year also!) Paula can honestly wish Ellen well, because she is not a replacement has-been pop princess. Ellen is her own brand. She's a totally new direction and no competition for Paula. This is very different than last year when Fox dropped Kara in to the mix.
This will also be good for Kara. Having both Kara and Paula seemed redundant.
Keep in mind that the job description here isn't really "judge". In the finals, all these folks do is comment, so they can more accurately be called "music critics" that give opinions and feedback. I'd love to see Ellen deflate some of the self-importance that often emanates AI judging table.