Since there are only so many cards left from Magics earliest days, the rarity of a highly played card drives up the price
For a mill deck to be viable in Commander, it has to target all opponents and turn mill into damage or some other benefit, and you should aim for big/infinite combos when possible
Tamiyo absolutely dominates a battlefield filled with creatures, and if the format revolves around creatures, then this rating may end up way too low
Indestructibility Simple and straightforward, Indestructibility is an aura that makes one permanent well, indestructible
After all, the more colors your commander has, the greater are your options when putting together your deck
Preston Garvey, Minuteman is another excellent option as your Commander in this deck too