[NickyBoy]

I doubt the majority of the listeners to Radio 2 will know, let alone care when Ken was due to finish or that he's finished a couple of weeks early.

If they wish they'll follow him to GHR, stay with Radio 2 or find something else to listen to. The world hasn't come to an end.

I agree, but the decision has made more of an issue out of it than it would have been.

As Ken said “it’s a bit of a shame for the sake of 17 days”.

I think it's more to do with the fact that Ken's face is all over Greatest Hits Radio's social media and website. Simon Mayo also said a couple of weeks ago that he was gooing to be off air for a day beacuase he was filming a TV advert with Ken, which when he is still employed by the BBC is a bit off if you ask me. That TV advert would likely to be on air while Ken was still at the BBC as well. 
I think the BBC and Ken to an extent were a bit naive about the process, expecting Ken to finish on 23rd March or whenever it was and then a week later pop up on Greatest Hits Radio, witout any advertising from Bauer. 

In the past, when a big name was leaving, there was a bigger gap between the end of the BBC show and them starting on commercial stations. Chris Evans finished on Christmas Eve and wasn't on Virgin until the end of Janaury, which was pretty much the same with Graham Norton. Simon Mayo left in the December,  and his new show on Scala wasn't announced until Janaury & didn't start until March the following year.