Cersei had laid the blame for the destruction of King's Landing on Aegon and had called the Vale banners. House Lannister did not have enough men left to deal with the dornish and Aegon on its own. Petyr could maybe...help advise the new Lord Arryn, but he would have to constantly wrangle with the Lords Declarant, that hated his guts. Or he could go to Casterly Rock and rejoin the small council, which now had many openings.