At the Canadian Grand Prix press conference, Alonso said that McLaren-Honda has to win by September otherwise he'll be leaving the team at the end of the season. This statement comes to no surprise as the Spaniard has been stuck with Honda's unreliable and under powered engine unit in his F1 car.
Because of the said power units, it has rendered him non-competitive throughout the season. It is also the reason why Alonso decided to race in the Indy500, where he had a chance of winning, as compared to the more glamorous Monaco Grand Prix. The fact that McLaren-Honda is currently dead last in the Constructor's Championship and having a DNS or DNF per race does not help either.
“I joined this project because I wanted to be world champion and we are not in that position, so if you don't see things changing and you are not in a competitive position, maybe you change project. That's the only thing I can say now. Until I sit down with myself in September, October or whenever, after the summer, I cannot say anything 100 percent now. I am very open. The best decision hopefully I will take after the summer,” said Alonso.
Furthermore, if the F1 season pushes to a 25-race calendar, Alonso says that he will quit F1 altogether. Currently the F1 calendar has 20 races in a season with a winter and summer break. There have been reports however that the F1 bosses are aiming to have more than 21 races by 2019, along with a shortened winter break.
With the hopes of McLaren-Honda having a miracle turnaround slowly diminishing, it would not be a surprise to see Alonso head elsewhere. If not Alonso could possibly stay with McLaren, but run full-time as an IndyCar driver.