Even though Metal Gear Franchise has more sales than MegaMan, PacMan, and Street Fighter? Okay Snake is totally not on their level.
While I don't disagree on the point you're making, I do want to point out that the Pac-Man and Street Fighter franchises might be a little hard to compare to Metal Gear, since their arcade origins dramatically change things.
Pac-Man (arcade) only sold 400,000 units, but it had a gross revenue of $7.43 billion in 2017 USD (United States Dollars.) Ms. Pac-Man also sold only 125,000 units, but we don't know the overall revenue of that one. Including other games in the franchise, the Pac-Man arcade games made a whopping $11.4 billion USD for Namco.
Street Fighter II, similarly, sold 200,000 units, but had a gross revenue of $4.15 billion in 2017 USD.
Your point still stands, but the Pac-Man and Street Fighter franchises need to have their arcade origins considered when making a comparison to other franchises, as the medium's a bit different.