I know this will be hard for some people to understand but the NFL is not the be-all-end-all of sports. And they wouldn't be in Toronto! Hockey and baseball have a more established history there, and the Raptors' NBA Championship is still fresh in people's minds.
"Surely the NFL can overcome that!" you say. Except the evidence isn't there to support that. Again, the NFL is not the be-all-end-all of sports.
The NFL tried in Toronto. Really tried. The Bills' Toronto series was pushed HARD. How hard you may ask? I was living in London, Ontario at the time (three hours out from Toronto) and you couldn't escape the marketing. So I can only imagine the marketing blitz the city itself got.
What did it amount to? A Rogers Centre that wasn't even filled to capacity. And the Rogers Centre is small by NFL standards.
So there's no evidence the team would be a draw. In fact evidence suggests it would be middling for the market and near the bottom (if not the bottom) for the NFL long-term.
The Rogers Centre brings up the second reason it wouldn't work. The Rogers Centre is small by NFL standards, and wouldn't be a viable solution. It would be suitable as a short term venue while a new stadium was built, but even that is out the window.
The Toronto Argonauts, the CFL team, recently left Rogers to play at an expanded BMO Field across the street. This was done so the Blue Jays could install real grass in Rogers. Converting Rogers from baseball to football isn't the easy thing it used to be, and the Jays would SO not go for it after finally getting the place to themselves.
Related to Rogers Centre...point three of why the NFL shouldn't go to Toronto. There's no NFL-caliber stadium in the city, and no will to build one.
BMO Field was the last stadium the City of Toronto paid for, and that's going to be it for the foreseeable future. Not to say that Canadian locales are above paying for billionaires' pet projects, because events in Edmonton and Calgary prove we can stoop that low.
Thing is...those were hockey arenas. Meaning they're cheaper than the football palaces modern NFL stadiums have become and they were for teams in a sport that is practically a cultural institution in Canada. Which American football isn't (hell, Canadian football isn't).
The City of Toronto/Province of Ontario isn't going to pull a Las Vegas/Nevada and pour 700+ million into a football stadium. So short of finding a Kroenke style billionaire who's willing to fund it on his own dime (and pay Canadian taxes on top of that)? It's just not happening.
Point four. I'm not an Argos fan (go Ti-Cats!) but the Toronto Argonauts are older than every other team in the NFL. They're among the oldest continuous sports franchises in the world. Not only that, they anchor the CFL financially. Toronto is the economic centre of Canada and its largest city. Merely having Toronto in the CFL makes it viable to advertisers.
There is a very real chance that a NFL team in Toronto could kill the Argos, and a very real chance that killing the Argos kills the CFL. And it's a very real possibility that without the CFL the Canadian version of the game dies.
I'm not willing to sacrifice a team with the history of the Argos, or the CFL, or the Canadian version of the game for some middling NFL franchise the market doesn't need and metrics indicate it doesn't want.