Stupid new mastery system

Why does riot pre-pick what champs you should play to get whatever rewards the Mastery Milestone thingy provides? Is the rewards even worth it?

For context I need to play Tahm Kench, a champ I've tried in 2 quickmatches when he was on free rotation last. There are like 2 out of the bunch that are halfway relevant to me, champs i've played a lot previously but i don't actually enjoy playing at the moment.

To summarize i believe not even 1% of the player base worldwide actually completes even half of the champs in their "milestone rotation" and if that's actually the case then the system is stupid and should be adjusted