I don't want 33 teams, never mind 40.
You're gonna get diminishing returns the more you water down the league. All the previously mentioned points are valid. There are too many bad QBs as it is. There are too many deeply entrenched fan bases to just keep throwing teams into every metro area. The international cities are too far away, how is a London team going to stay competitive playing overseas 8 - 9 games a year, or when it plays against a western division and has to travel across 9 time zones?
Another issue is fan hopelessness and loss of interest. 2021 has been a great year for parity so far, but how many of these teams really have a chance? By the time the divisional round arrives, won't we be looking at the same list of usual suspects - Kansas City, New England, Buffalo, Baltimore, Green Bay, The Rams, Tom Brady? Parity in this league is a myth. Wins are monopolized by a small cluster of well run, well coached organizations. Every year or two a new team crashes the party, but it is a slow, slow turnover. The flipside is the losing, in the 21st century NFL a team is just likely to find itself in a decade or more of soul-crushing ineptitude as it is to win a Super Bowl, and that's with 32 teams. Reality is, the next decade of this league is going to be owned by Brady, Mahomes, Jackson, Jones, maybe Kyler Murray. Everyone else is an also ran.
The league has already had to relocate 3 teams over the past 5 years, and Buffalo was nearly a fourth until they finally got it together. Are fans in London, or San Antonio, or St. Louis going to keep watching when their teams go 25 -94 over their first seven years?