I think it proves its not an infeasible business model. The Wolfpack didn't need on field support to be a success (I'd say the majority of Torontorians have never heard of the team, and the only significant exposure they've got outside rugby supporters is coverage by Tim & Sid who seem to be fans). Air Transat was vital, but I doubt the Wolfpack even exist without an airline sponsor. Not to mention, it's not really a burden for the airline anyway.
Theoretically, it should be able to be scaled up to any leagues similar to the Wolfpack without major issues I get the emotional opposition to a London NFL team, but I don't get it business wise. NFL teams are massive money makers, and that's regardless of whether a team sells out or not. A London team opens itself up to over 500,000,000 people as a potential audience. Realistically, only the UK and Ireland (perhaps Germany/France as well) can be counted on to be legitimate markets, but that's still over 70 mil alone in the British isles. Your average NFL game was watched by 16.5 mil people in 2016 (a significant drop from 2015, but still huge). I don't imagine a London team would bet significantly less than that, and there's potential for it to be substantially more, if games are timed properly (London games at night, away games in the very early afternoon/late morning, and only play on Saturday/Sunday).
The owner of the Wolfpack has suggested that transatlantic teams are feasible for all North American and European sports leagues, but I don't agree yet. You can't transpose MLB, NHL, or NBA seasons across the ocean unless the market for those sports also crosses the ocean (none of these sports have flexible enough schedules to make sure cross-Atlantic games are still viewable at reasonable times). I don't think the flight times is as big an issue; by next year the XB-1 (a ~40-60 seat supersonic jet that flies faster than the Concorde) should begin it's first flight tests, and supersonic commercial aircraft should make a legitimate comeback by the end of the next decade. At ~3 hours from London/Paris to NYC, flight times are a non issue. It's just the game has to be global in scope, or you have to play such a limited number of games to make it work. I actually think European soccer has the best potential for a transatlantic team in a major sport, but the way the sport works in Europe makes it very unlikely.