Fernando Alonso is one of-if not the fastest drivers over a race distance of this modern Formula One era. His consistent speed and mental ability has left many in the dust, or in Lewis Hamilton's case, to another under performing team at the time, Renault, that manged to get a win out of the Singapore crash-gate fiasco. So he moves to Ferrari, does well right off the bat, and then doesn't like the culture. Boom! He's off to McLaren for 2015.
Everyone knows by now that McLaren have been relatively nowhere compared to what their in-house capabilities are. So, Honda is off to Red Bull junior, Toro Rosso, leaving Red Bull with only a Honda engine for 2019, as Renault was sour from their past engine issue mudfest, and dropped them. Christian Horner is was sad at first, but now it turns out, they will eventually sell out mostly all assets to Aston Martin, and will have an Aston-badged engine very soon.
Is Renault going to make a large enough jump next year to convince Fernando to stay? I think so. The Renault powertrain package is a much more efficient package than Honda with tighter packaging, and then McLaren can do their magic. But probably a year long deal. Maybe a LeMans entry is on the cards too. IndyCar has great racing, but they aren't F1 cars.
Should he?
I'd say he should stick around one more year at McLaren, and there will be many seats open in 2019...but hey anything can happen!