If there's a cost associated with having that creature attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.
And when you give a card a bunch of effects that are normally situational, that can be especially powerful because when those scenarios come up, you get insane value
These combos can be performed very fast and, more problematically, Lumra can easily go for another win after a foiled attempt
He top decks a Snuff Out, removes the untapped elephant and I die the next turn
Redundancy and quick diminishing returns hurt the value of those groups of cards
For just three mana, you can give each opponent a poison counter, starting a very short clock for an alternate win condition