If you are Unity, you have to do charity, and offer your products for free, and never try to sell anything else :^^)
Let's put things into perspective shall we:
Facepunch sold 16 millions copies of Rust in 2023 [1]
At $40, it's ~448 millions USD after the 30% cut from Valve (30% flat tax lol), $500k a year is not even 1%
But wait a minute, what even is Rust? Rust is an online open world PvP survival multiplayer game, so it relies on a cloud infrastructure with beafy servers to handle all the physics and proc gen, if they sold 16m copies just in 2023, they then have to handle a lot of concurrent players[2], whether it is from Unity or not it doesn't matter, so the 500k is relative, it's a successful game, the invoice will be sweet regardless of the cloud provider
Without knowing more details, the email probably goes: "We now bundle the product X, and you meet the criteria"
Remember, less than 1%, and is relative to the type of game and its success, shy of the 30% cut from Valve
I don't particularly like Unity, but people are free to use the engine of their choice, or roll their own
For steam however, it's where the users are, therefore you have no other choice but to get taxed 30%
So make sure you fight the right battle, nobody forced you to pick Unity, you had choice