I'm not brilliantly versed in the machinations of the NFL but surely the key for a new franchise is as much making sure that it's competitive (and quickly) as much as ensuring that the raw interest is there?
To use a football parallel, I'm sure playing Man Utd vs Liverpool in Singapore would attract an absolute huge audience. Similarly Wembley gets good crowds now when the NFL teams pay an annual visit. But if there were a Singapore Utd parachuted into the Premier League and they were rock bottom and lost all the time, crowds would dwindle very quickly. If you created a London franchise in the NFL and they struggled then I think you'd soon end up with a very sparsely populated stadium (and as Wembley is so big, you only need a relatively small drop off for it to look horrendous, have a look at the crowd tonight which all told for a game which is as meaningless as it gets, won't really be that bad).
Identity is pretty key here for sports fans too I think - franchising is not a familiar concept. There are many who like me, struggle to accept MK Dons over a decade since they moved* the club.
*Stole.