These cards specifically state toughness somewhere in their text and use the value of a cards toughness in their effects
Card Name Mana Cost Type Legendary Artifact Rarity Mythic Rare rules Text IndestructibleWhen The One Ring enters, if you cast it, you gain protection from everything until your next turn.At the beginning of your upkeep, you lose 1 life for each burden counter on The One Ring.: Put a burden counter on The One Ring, then draw a card for each burden counter on The One Ring
Regardless, I dont know that now is the time for more than 60 cards, but I do know that as the card pool expands, it gets easier and easier to find more spells of a similar power level, but no other lands are close to the power level of fetchlands, so the incentives to build to maximize the power of our lands over the power of our spells do increase in some way over time, which is to say that, even if its not right now, it could be someday, and getting there first might be the best way to truly break Modern.
Its a drawback that needs to be considered when playing this mechanic
Silver Border Black border this, white border that
Or the worst of the worst, I suppose