are we getting higher mastery scores?

It's difficult to get 3 'S' ratings unless youre actually good.

I feel like we will because 7 is a weird number to stop at.

I expect mastery 8 requiring X S+ grades would be the next logical step.

It's unclear how they'd go about mastery 9+ requirements.

They also don't want to rush into it, as they want to make sure there are suitably exciting rewards that are also practical (so a unique skin for every champ is almost certain to not happen for example).