Alonso isn't totally washed up yet. He'd have to work his way down through NASCAR and a couple of years as color commentator on F1 broadcasts before he gets to Formula E.
When the World Hockey Association first formed, they were trying to gain 'credibility' against the established National Hockey League. Mostly they hired kids too young for the NHL (including Wayne Gretzky), or washed up NHL players (Gordie Howe). Everyone realized they were just taking the leftovers, so the WHA pooled their resources and all the teams combined to pay one player a million dollars. They chose Bobby Hull from the Chicago Black Hawks, and he became the first million dollar hockey player, and the 'face' of the World Hockey Association.
If Formula E wants to be seen as a top series (that's a big if, I get the feeling they're happy where they are), then giving Alonso anything he wants would be a nice thought, instead of waiting until he is washed up.