They need to stay for continuity and they are both very good at their jobs. Look at X Factor for example: it's suffered in the long term with having a different panel every year since 2011. Also, losing an XF staple such as Dermot (who many on DS thought he'd become stale) has been a disaster for the show since they've brought in two presenters who were car crash TV. Olly Murs is proof that not everyone can present but both Emma and Rylan can - whether you personally like them or not.