All the evidence? The successful international series combined with the population... Yes, if you ignore all valid arguments maybe you can make an argument that London isn't viable, but at the end of the day, you'll still be wrong...
Perhaps it's because you're confusing it with the idea that football will somehow takeover as the premier sport there... No one is saying that. But can a city with 8 million people who have packed stadiums in each opportunity support 1 NFL team better than say... the Jacksonville Jaguars who are one of the smallest markets in pro football... Yeah, they can....
So in order to support an NFL team and all the competitive balances that are required how do you expect to find 100,000 people who are that invested in the game to pay top dollar? And I mean top dollar because you have competitive balances you have to adjust for. Taxes, schedule, practice days lost on flights, personal travel for players, health risk for extended periods in the air, cost of second homes and currency exchange.
(Currency exchange means if you are paid in pounds you want guarantees to translated value in US dollars since you are a US citizen and claim primary residence in the US. You don't want to get paid in British Sterling only to find your 15th and 16th paycheck in your contract is devalued because the pound is devalued.
At least this is what I would demand if I was agent acting on behalf of my US born player who tells me when he retires he will be living in the US.)
All competitive imbalances have to be borne by someone. The owner, the fans or typically the paying customer.
Do you really have an argument to prove that the game of American football truly resides in the psyche of the British citizens as much more than a curiosity?