A lot to unpack in this post.
First of all, Netflix is very sustainable. They are still in their growth phase but soon they'll have so much content they won't need to be spending as much on new stuff. There will be more than any reasonable person can get through.
Second, he's not saying that Game Pass is just a place for MS to dump money into. He's saying it's not sustainable because that implies just maintaining a certain level, he's saying it's actually more than sustainable, it's actually a growth business.
Being sustainable = being profitable at current level without needing growth.
It's clearly not, not at 23million subscribers and with so many discounts to attract customers. But that's not what Gamepass is meant to be at the moment, as already discussed before.
The reason why MS is dumping all that money in Gamepass is to reach that point you are describing Netflix is just barely reaching (they did $500m profit on 6.6b revenue last quarter I checked), and to overtake/ kill the competition (I don't buy it is solely a move to grow the industry, though it may do that).
Netflix just now arrived to a point where they don't need to borrow anymore ($16billions in 10 years) but they also need retention, which will be a key factor all ngside offering more owned content than contracted, specially as competing services arrive.
And more growth still, even with north of 200million subscribers.
MS's asset here is that obviously they don't need to borrow the money to finance Gamepass.