People are already paying 6 mana for a 3/2 vigilance in their control decks, it basically doesn't matter how bad the 8th card you get is: it's an extra card.
You
can balance things like this, you just have them replace a card in your opener instead of being an additional card.
Eg:
View attachment 8103
MSEM has a few of these. They use different deckbuilding constraints than companions can, because it's a digital format. Companions have to use deckbuilding constraints that can be disproven by a single cardplay.
And yes, just in case anyone was wondering, if there isn't any cost to adding your 8th card or making your opening 7 better, it's free and now everyone has to do it.