I've changed my mind. Or more accurately, refined it.
"Assuming you're going to win" is a decent heuristic for determining what games you want to play, because you ask yourself:
"Would I even be proud of the outcome, if I'm destined to succeed?"
On the other hand, by assuming you're going to lose, you ask yourself:
"Would I do this thing, even if I'm doomed to fail?"
I think both are questions worth asking, but the latter one, might be more valuable.
Intuitively, it's because of the ol' Emerson adage: "Life is a journey, not a destination".
But thinking a bit deeper into the clichè, it's also because having an outcome-independent mindset gives you a real, technical advantage.
It's the guy at the poker table just there to have fun (whether because he's got a huge bankroll or just a skilled veteran on his day off) that can easily stay composed making huge bluffs.
The very fact, that regardless of whether he wins or loses the hand, he's having a great fucking time, makes him terrifying to face off against.
It's in a sense, an "optimistic fatalism".
We're all going to end up in a grave somewhere, and thus we always end with a loss. What do we want to do in the meantime, in spite of this inevitable fate?
You can also extend this principle from these big life decisions and armchair philosophiz'n, to more specific, practical choices.
For the degenerate crowd:
What would you invest in, even if they could never return you a profit?
For the artists:
What would you make, regardless who noticed or appreciated?
What songs would you sing, regardless of who listened?
What books would you write, regardless of who read?