Power creep

Power creep is a phenomenon present in any collectible game that uses both old elements and new ones. It is the gradual unbalancing of a game due to successive releases of new content.

Description
The idea behind the concept is that a company has to sell its new products, but everything new they create has to compete with previously existing pieces. To compensate for this, cards end up becoming superior to other cards to the point of becoming strictly better. This means that older content becomes obsolete or relatively underpowered. Perceived power creep in Magic is often a source for complaints of the player base, predicting the end of the game.

According to Mark Rosewater, each set has things that go up in power while other things are brought down in power. This creates the illusion that the power is always going up because the focus is where they are pushing the power. This technique helps to create the illusion of the power increasing when, in fact, it actually stays pretty even. R&D actively endeavors to keep the power level of the game consistent. While there will be a variance from year to year (Mirrodin block was more powerful than Kamigawa block), the overall power level of the game is kept even. This is important because an unchecked power level will eventually spiral out of control and indeed kill the game. Second, power creep negates the value of older cards. Third, it plays havoc with design space.

However, it's no secret that creatures are better now relative to other card types than they were ten or fifteen or twenty years ago. As a consequence, the best creatures of the past are now mediocre. For example, Serra Angel used to be the star of the first core sets but was later changed from rare to uncommon and eventually ended up as an extra in the Magic 2015 sample decks. Creatures as a whole aren't becoming demonstrably overpowered, but there must always be a generous handful of pushed creatures in every set according to current design principles. This is because of the imbalance in power between creatures and spells that Alpha introduced. Since Alpha, there has been an ongoing effort to make creatures stronger in comparison to non-permanents.

One of the tasks of the Play Design team is to prevent power creep. R&D calls it the Escher Stairwell, where they lower one aspect while raising others, so it appears like it keeps going up while actually staying steady.