That's where backup strategies are necessary
For the entire history of Magic The Gathering, Legacy goblin decks were mono-red
Legends also got a few cards restricted for actual reasons: Recall: At the time, it kind of made sense because it made it a little too easy to reuse all your powerful restricted cards
Even if the card isn't a Rat itself, if it makes Rat tokens it also goes in, like Piper of the Swarm, Ogre Slumlord, and Chittering Witch
Are they going to be worth the price of a Happy Meal
Finding out which cards are the most valuable also tells you which cards are most important, and discovering what makes those cards worth their asking price is an easy way to understand which concepts are powerful and universal to the game of Magic