Surely Heart’s core audience is women in their 30 and 40s they’ll be familiar with Jamie. Also it is continuity for their largest licence so that important chunk of the audience that should remain stable.
Global's corporate website has Heart's core audience of 25-44 at 42%.
However it has skewed towards bringing in the next generation of that audience in by skewing younger. This has been achieved by replacing the likes of Toby Anstis with the younger Pandora and Sian Welby, a former weather girl on Channel 5, along with the already mentioned scrapping of 80s music and the majority of 90s music, going for a core 00s and 10s playlist. A digital station, Heart 80s was launched to move those older listeners across.
So when you bring in the mother hen delivery of Amanda Holden and keeping Jamie Theakston, it goes against what they've been doing to the rest of the station.