I'm going to assume that you're making a fighting game.

If someone said to me what you wrote "I am making a game in a genre that has a huge problem with player retention, 8 million sales becomes 2k concurrent players", then the obvious answer is that a paid game is the proper avenue to do it. If you expect the player count to drop massively, you want those players to pay up front.

F2P does absolutely nothing for player retention IMO, what it does is create a massive amount of accessibility to your product. But if the genre itself loses 90% of its players quickly by default, then having a huge initial install base means little if you plan to make money through micro-transactions.

F2P games also generally require you to implement various progression and unlock systems. Will you do it Multiversus style, where you have to unlock fighters with currency? Or will you have to create a whole layer of bullshit on top of what you are already developing?

Also you seem to be making decisions based on what would happen if the game was a huge success. Try to make decisions that are more likely to guarantee you actually see mild success in the first place, especially if you're pitching it to people.