It's a free hit imo - failing at Chelsea isn't a blot on Alonso's CV, everyone knows the club is run by idiots. If he went to Liverpool or City and failed then it would probably stop him from ever managing in England again at the top level.
Whereas if he gets Chelsea back in the CL, which I think is do-able, it'll be viewed as a success and Alonso gets some Premier league managerial experience too.
And even If it all goes to shit he gets a £10m payout (or whatever) and a chance to try again.