This time it may be a few days but the next piece of DLC or worse, a full game is exclusive to GP for a week, then a couple weeks, then a whole month.
This type of actions make me want to unsubscribe to the service altogether.
I don't think that's the case. If it were, they wouldn't have referenced Xbox owners having access to it "soon".
While I totally see what you're saying, I'd be a lot more concerned if it weren't Xbox branded cosmetics and an obvious promotional thing. Maybe it's a slippery slope, but considering it's obviously something Xbox asked Psyonix to do and probably worked with them to do it, I'd say it's a bit early. Any DLC comes to game pass as a timed exclusive that isn't Xbox branded cosmetics and I'll be right there with you though.