If the game is fun, people will play it. It doesn't have to be overly-challenging or extremely grindy for it to be addictive to players, they will come to your world if they like the gameplay. The top selling game of all time is still currently Minecraft, and you couldn't really say that is a particularly tough game to play, or hard to grind out materials. It can be made to be challenging, if desired and is nearly infinitely mod-able. Take a look at Pokemon: it sells extremely well each year it gets released, very easy to play and collect the monsters isn't a massive grind. Players can impose their own hardcore challenges and the PvP side of the game has their own tournaments with meta brackets. More relevantly, I have been playing a lot of Baldur's Gate 3 lately while spending less time in Destiny 2. With so many Origin paths and many different ways to overcome obstacles, it easily is a game that will last for hundreds if not thousands of hours. "Honour" mode exists if people want a challenge, but even on "Balanced" it is enjoyable with a level in-between and an easy mode if they just want the story. But hey, add some more items to the Eververse store and maybe you'll keep afloat for a couple more months while player counts keep plummeting.