Even with Rocket League microtransactions are a bad idea. It incentivizes the developer to make attaining items through gameplay a slog in order to get you to skip that by paying up. If there was no way to get those items through mtx, they'd make the process of getting them fun.
Also, games are supposed to be a form of escapism, where the gameplay matters, not who you are and how much money your parents make.
You can argue in favor of the game itself being free allowing people to save 10 or 20 bucks. But gameplaywise Microtransactions inevitably make the gameplay experience worse, without exceptions.
Anectdata: I have over 2000 hours of Rocket League, never once spent any money on it after purchasing it and never one have I felt that my fun was impacted by the available microtransactions.
Nothing that can be purchased in the game affects gameplay, it's all just cosmetics.
It seems to be the most common item people buy (because it can actually slightly affect your performance as it better matches the hitbox, and it is the most used competitively). I know a lot of people who never bought a single item, except for the fennec.
The fennec hitbox is the same as the Octane, but the design of the car better matches the hitbox compared to the Octane. So you can be more precise with your touches.
Also, games are supposed to be a form of escapism, where the gameplay matters, not who you are and how much money your parents make.
You can argue in favor of the game itself being free allowing people to save 10 or 20 bucks. But gameplaywise Microtransactions inevitably make the gameplay experience worse, without exceptions.