Of course Alonso is in a very comfortable situation. But things can change fast. I think it will depend on how many cars will be able to fight for victory in the next races.
If the Williams and Ferraris manage to improve (which is possible, especially for Ferrari who just needs to make the tires work better), they could play a role in the battle between Alonso and Kimi.
The advantage of Fernando is that even if Kimi is faster by far, he still can secure 2nd or 3rd place. Now imagine that the Ferrari suddenly become faster than the Renaults, but not faster than the Mac Laren. That could help Kimi.
On the other side, the Ferrari could also win the race instead of kimi, helping Alonso.
So I think that there are several possible endings to this story. JPM has also a role to play, but at the moment he didn't, having several issues that took him out of the lead group.
We also have to notice that for the moment, Alonso had 100% of chance with him. His last race could have been ruined at the 1st corner (when he was hit by Ralf), but he finally won. In monaco his tires were ready to blow, but they lasted long enough. On the other side, kimi was very unlucky. Now, what next if the chance decide to choose another guy ? What next if Renault's reliabilty issues start to affect Alonso instead of Fisico each time ?
I remember Michael in 2000, he had a 30pts margin over Mika after the Canada. Everybody said it was over. Then he crashed Twice, had a failure, and his lead was blown away after Hungary. Then he lost the lead in Belgium. And he finally won the last 4 races, after several people said it was over for him.
Everything's still possible, but of course, it's better to be in Alonso's place right now.