It wouldn't completely kill the genre, but seeing the percentages would definitely turn off some people, leading to less profit.
For example, in my made-up lootbox game, I say that you can buy this golden chest for $5 and you can get the strongest character in the game. Well, sounds pretty good right? But I never said you will get it, just that you can.
What if, then, I have to reveal to you that the chance to get the strongest character in the game is 0.1%. Does that sound enticing anymore? Probably not, considering on average, the strongest character in the game appears in 1000 treasures. That's $5000 on average to get the most OP character in the game. That's an average.
Also, a lot of these types of games only give you a total percentage to make things look nicer. For instance, let's say that the characters in my game are separated into copper, bronze, silver, gold, and rainbow ranks. The chance for a single rainbow character is 5%.
Sounds OK right?
But then what if I told you that that there are 50 characters in that 5%. Well, if there's a specific character you want, you have a 0.1% chance of getting that one character. In a lot of these games, a good number of these rainbow characters aren't even good, meaning you're wasting tons of money to get garbage.
Knowing that your chance of getting a good rainbow character is 0.1%, is an incredible turn off. Those games don't want you to see that. They just want you to see the "5%."
I think the worst of all is that you're not guaranteed to get that character.