When this subject came up last year, I did some research and saw her show had been on the air for 17 years. We have to acknowledge that's an extraordinary amount of time for this genre of daytime talk show. Most of these shows last only a year or two. So when a phenomenon like Ellen shows up the network or syndicator wants to keep the show going at all costs and at each contract negotiation the host gets lots more money and more power. So after 17 years Ellen is getting tons of money to continue. Maybe she's burnt out - when Craig Ferguson made his announcement of leaving the Late Late Show he said he felt like he'd done all he wanted a couple of years before and when he told CBS they gave him an incentive to stay and he did a couple more years without making much of an effort.
The other part of those deals is that the show has to continue to put up high numbers. If an agent is pushing Actress X to be the new hot daytime talk show host the network/syndicator has to make the decision: keep Ellen with her high costs and solid ratings or take a chance with X who will come much,much cheaper but might take a long time to reach Ellen ratings or might not ever reach them. With each new contract the pressure rises as the money grows and with the ratings drop they might have come to the point to go cheap and take a chance on a new show.
I don't doubt there is some level of burnout with Ellen. The world in general, especially technology and the entertainment business have gone through massive sea changes in the 17 or 18 years she's had her show and keeping up became overwhelming. I won't speculate who initiated ending the show but I don't think it was a big surprise to either side and the side that didn't initiate was relieved.