Like most Magic: The Gathering cards, the value of Rift depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Mystery Booster Playtest Cards 2019)
There were only two downsides Seismitoad had: First, the damage output was not very high, so it took time to KO an opponents Pokmon and, second, the fact that Virizion Genesect was one of the most powerful decks in Standard
Playing Winter Orb with Urza, Lord High Artificer is a game plan
But for most of the part it will be quickly forgotten and drop in price heavily
Even with myriad you're still only getting a temporary advantage, so you should make sure you either borrow something that's going to leave a mark, or lean on sacrifice effects to circumvent giving the creature back