It's not that simple. Grimmy is actually doing a good job when you take a step back. He's exactly the sort of presenter they need on breakfast to attract younger audiences and, whatever we may think of him, he appeals to them. "Grimmy Loses More Listeners But Listening Among Young People Across The Board Is Actually Down" isn't quite so catchy as a headline.
Moyles had more listeners, yes. But a lot of his listeners were over 25 and part of a generation who grew up with Radio 1 and listening to the radio as a habit- it was a main source of entertainment and music. Grimmy was basically told by Ben Cooper (who was put under huge pressure by the BBC Trust to lower the average listener age) to ditch those loyal over 25s and focus on the 16-24 demographic. That generation has YouTube, Spotify, Buzzfeed, Facebook, Instagram, the list goes on- while they're on those sites/apps, they're probably not listening to the radio. Radio 1 is in a constant fight to attract them by going where they are and using video/ social media etc to get them to listen. The fact that Grimmy has added so many young listeners is quite an achievement IMO.
I'm not saying he's one of my favourite presenters on the station (he's not, I think Scott and Greg are much better and would prefer them on breakfast) but if I was Ben Cooper, I'd be struggling to find a major reason to get rid of him right now.
Suggest people give this a read : http://www.nme.com/blogs/nme-blogs/mythbusting-radio-1-losing-500000-listeners-half-story-1971412 .
It's balanced, fair and provides some good insight into where radio aimed at young people is going and how Radio 1 will likely be shaping it.