Like the time he left Ferrari and Ferrari got better under Vettel/new management
It's not like that happened instantly, Alonso almost pulled off something we had never seen before. A championship in neither the fastest or most reliable car at Ferrari. It's understandable after years of frustration dragging Ferrari where it should not have been that he sought other avenues. It's not like he'd have another title had he stayed. He took a gamble and in the scheme or world titles he's hasn't missed out on any.