While they will likely have significant impact on Standard by serving as sideboard safety valves to keep graveyard, energy, and +1/+1 counters decks from getting too powerful and ruining the format, the bigger deal here is that they signal good things for Standard's future
Cards like Celestine, the Living Saint carry prices well above what their effects would normally justify, purely because supply was absorbed early and the licensing makes reprinting them genuinely difficult
Between enter the battlefield effects, triggered abilities, and other synergies, it's often the case that playing a creature provides an opponent with much more value than just the creature's board presence
Although this card only comes as a promo, it is only legal in EDH and Vintage so demand isnt super high and its price isnt too out of control
If it lands on tails, the attack fails