A lot of games are designed following a model which you could call a core shell model: There is some core activity of gameplay (which, given how prominent violence is in games, is often a battle), and then a shell around it, which has everything else. The shell often has progression elements, like management of experience points and levels, talent or tech trees, and sometimes non-progressions elements like the story. A typical player in a typical game of this sort constantly jumps between the core and the shell: Does a battle in the core game, then levels up his character in the shell game, to go back to the core battle with a stronger character.

Core and shell are fundamentally different, and only the core is viable on its own. You could imagine many games like Fortnite, League of Legends, or World of Tanks with only the core, and no shell. In World of Tanks you could theoretically just have tank battles, without any tech tree progression, any tank equipment research, any crew skill management, and the like. In fact, since I tweaked some settings in my mod pack for World of Tanks, I now get a display at the start of a battle which shows how many battles in that particular tank each player already did. And I am always surprised that some people play thousands of battles in the same tank. I recently saw a guy who had twice the number of battles in this one tank than I had with all of my tanks together. I assume that these players just like to play this favorite tank of theirs, and don't care about the shell elements of World of Tanks. After a certain number of battles you have the best possible equipment, and your crew skill development has slowed to a crawl. The shell of World of Tanks has events, and features like "2x (or even up to 5x) experience points for the first win in each tank" that encourage people to play many different tanks. But doing just the core battle always with the same tank clearly works as well. Having no core wouldn't work, even idle games but in fake battles with little or no interaction instead of skipping the core totally.

Regarding the recent discussion on paying to win, or paying for power, for me it makes a big difference whether you pay for more power in the core game, or in the shell game. World of Tanks has pretty much shifted all the power you can pay for into the shell game, you can pay for faster advancement, or for getting tanks directly for cash. The separation isn't 100% clean: The so-called "gold" or "premium" ammo, although not buyable directly with gold any more, are still much easier to afford with a premium account or after farming credits with a premium tank. And premium ammo gives a definitive advantage in the core battle game. And some people would argue that some of the premium tanks are "overpowered", although I must say that most of the premium tanks are definitively underwhelming. (I don't buy premium tanks any more unless people complain about it being too powerful, like the Defender currently on offer.)

But even if the separation was complete, the question would remain what part of the game is really "the game", and what is a "win" in the term Pay2Win. Quite a lot of games have some mechanic where paying results in faster advancement in the shell of the game. As being of higher level or tier then usually doesn't give you an advantage in the core game, because you are simply matched against other players of the same level, faster progression doesn't really make you "win" more. But then some people consider the progression part of the game, in the shell, more important than winning or losing battles, so faster progression is exactly the kind of "win" they want. At least in World of Tanks I had exactly the opposite experience, that playing at higher tiers was less fun for me than playing at lower to mid tiers. If I am paying for faster progression (which I am, in the form of the premium account), it is more to be able to unlock more different tanks and tank lines faster, than to reach tier 10 in one line (which I rarely do).

One thing World of Tanks doesn't offer, which would be the completely opposite idea, would be for players to be able to pay for guaranteed top tier in all their battles. Now *that* would really feel like paying to win for me, and thus not really fair. The top tier tanks tend to dominate in a battle, or at least are much more likely to than the bottom tier tanks. If you listen to the chat, you can always hear people complaining about being bottom tier tank again, even after the recent matchmaker changes, so I'd guess this option would sell very well. But I guess in the interest of the game being perceived as fair even to free players, this isn't really a viable option to introduce.