Probably two or three more Splatfests.
For what it's worth, I'd like a new Splatoon next year. This isn't Mario Kart. Franchises in this genre aren't left to go dormant. Splatoon has a competitive scene and a world championship. It needs to stay 'fresh'. You either support the current game or release a new one. You don't wait for Switch's successor and let the momentum die for a couple of years.
For what it's worth, I'd like a new Splatoon next year. This isn't Mario Kart. Franchises in this genre aren't left to go dormant. Splatoon has a competitive scene and a world championship. It needs to stay 'fresh'. You either support the current game or release a new one. You don't wait for Switch's successor and let the momentum die for a couple of years.