6 mana is a lot to ask for a mana doubler in the modern day, and while the ability to turn that mana into power is nice, the lack of evasion really hurts this cards ability to serve as a reliable finisher, even with an Urborg, Tomb of Yawgmoth in play
In casual games, it's considered good manners to do so
Super-Skrull Yes, a commander with an activated ability for multiple colors is strong, but Super-Skrull is notably less dangerous than Kenrith, the Returned King
Sire of Seven Deaths $29.12 Sire of Seven Deaths sports a keyword soup of seven keyword abilities to go along with its 7 mana value and its 7/7 power/toughness
I can see this card being a nice commander or a good addition to combo decks
At 7 mana, it's certainly too steep a cost, but I love the design