I can't help but feel Susanna Reid has lost her mind with this move.
I suspect she's banking on the GMB position being relatively short/mid term, actually. Say she stayed at the BBC: what would she have done next? There's not a difference between being a Fri-Sun presenter and being a Mon-Wed one (except on lifestyle), which means Susanna's had the same job in effect for a decade. She doesn't strike me as someone destined for hard news, and the likelihood is Sian and Kate would still be presenting had the programme not moved to Salford.
She spends a couple of years at ITV's breakfast show and, assuming it's unsuccessful, gets moved to some other form of programming. That's a scenario she likely wouldn't have had at the BBC. ITV seem to work quite hard to find their presenters new roles when they want to start again - Kate Garraway has been kept on for a long time since being dropped as a main presenter, while Adrian Chiles, Christine Bleakley, Lorraine Kelly, Aled Jones, John Stapleton and Lucy Verasamy have all been given new positions or deals. If ITV really do see a value in Susanna beyond Breakfast newsreader, my guess would be she's banking on getting the same treatment.
Also, while I don't endorse some posters' talk of her personal life, I do wonder if money might have had something to do with her move. If I remember correctly from interviews her train fare and hotel were being paid by the BBC for around two years after the move to Salford as part of the relocation package - a couple of years on, I wonder if she'd now have to pay those expenses herself? My guess is that would be rather pricey, and even having her existing deal matched at ITV might, if I'm correct, have been worth a lot more as a London based position.
Re: replacement - I wonder about a return from Kate Silverton. I make no comment on how such an arrangement may or may not work for her lifestyle and family, but when Sian Williams returns from sabbatical (and Silverton from maternity leave) the network news team are again going to be overstaffed. Kate is familiar to the audience, has 'star recognition' compared to Louise and Naga, and might be seen (at least in the short term) as a good rebuke to any offering from ITV. When Daybreak first launched Breakfast had a very stable, confident team in place, and the 2012 relaunch saw familiar faces up against familiar faces. This time, with ITV poaching one of their own and forcing at least a small degree of change at Breakfast, the BBC might look to a bigger name (for insurance) as Greg suggested.
One other thing - I hope they get rid of "pic'n'mix" Sundays. Not a fan.