Right now even half decent players run around with mastery level 5 or 6; because mastery never resets this means less and less. Back when it was first released seeing a level 7 player comforted me; now it means basically nothing. Maybe riot should add infinite levels or soft rework but eventually everybody will have mastery 5 on every champion.
Mastery isn't supposed to show how good you are at a champion in relation to other players, it shows time spent playing a certain champion, which means familiarity, and a higher likelihood of proficiency.
So yeah, you play the game long enough, you spend enough time with every champion, you deserve to have that mastery on every champion.
The only place I think a reset is in order is with full VGUs, as many champs change quite significantly (Aatrox being a prime example), so that metric is no longer true.
