I was going to post this last night, it seems nowadays when someone leaves a show the lazy option is guest hosts. Guest Hosts also didn't work on Channel 4 5 O'Clock Show which replaced Paul O'Grady and most weeks doesn't work on Saturday Kitchen therefore I would of liked them to give the show to someone and stick with them for the whole 8 week run.
It's hardly a lazy option, though, it's surely far harder work to have guest hosts all the time, because you've got to find a new one every week, and they've got to be trained up in how the format works. It's much lazier to just do the same thing every week.
It seems more likely that shows use them now because it allows them time to do what's basically on-air auditions without immediately plunging one in and potentially dying on their arse, especially if they're replacing someone very popular. The Five O'Clock Show was always doomed to failure, it was a ridiculous idea because the show was so associated with Paul O'Grady.
Problem is getting someone to agree to host 5 days a week every week might be difficult unless its someone new who hasn't had their own chat show yet. 5 nights a week would be a big commitment.
Reminds me of Jack Docherty when he started his show saying "It would be good if someone said, I'm thirty now and I'll do it until I retire. But it won't be me." You say it's a big commitment but you have Uncle Matt and Alex doing the One Show five days a week (I know Matt only does four days, but you know what I mean). The difference there is that you're not eating up comedic ideas quite so much. The One Show sort of runs itself.