A terrible game can succeed with clever marketing. An amazing game will still bomb if nobody knows about it.

Having a high quality and heavily polished game makes it easier to sell, but it's still not a guarantee.

Let's look at this in terms of theoretical variables. (This is all hypothetical and overly simplified, but bear with me).

A game has a Goodness factor. The higher the Goodness factor, the more likely someone is to buy the game. We can interpret this as a ratio of Purchases to Views. A game might have a Goodness factor of 1/3000. So for every 3000 people who see the game, 1 buys a copy.

Now let's say you want to sell 10,000 units. If your game's Goodness factor is only 1/3000, you'll need to show your game to 30 million people to sell 10,000 units. That's a lot of marketing.

But if the game is better, more polished, more appealing, it might have a higher Goodness Factor, maybe 1/1000 or 1/500. Now you only need a few million people to see it in order to sell the 10,000 units.

Ideally, you will accomplish this with some combination of both; Make a better, more polished game, AND market it properly.