New York has the Roosevelt Island Tramway (Cable Car in US English) which was initially designed as an essential transport link, but then became somewhat of a major tourist attraction after it was replaced by a Subway connection.
Emirates are sponsoring this project to the tune of £36 million as well, and I very much doubt they would have sponsored a foot tunnel! I thought it sounded a bit gimmicky at first, but how was TfL going to get £36m of private money for a tunnel to provide another type of crossing from?