When Wizards of the Coast announced Magic: The Gathering's Secret Lair drop series last November, fans were worried it would result in a system where the company sold overpriced singles directly to consumers, cutting local game stores out of the picture entirely. This is true to some extent, but it was mitigated at the time by the nature of Secret Lair, which consisted solely of alternate-art treatments for popular cards. Players who didn't want them didn't have to buy them. They could stick to the regular versions of the same cards instead.
The release of Ikoria: Lair of Behemoths and its special alternate-art Kaiju cards further concerned fans, who worried that the game would be transformed beyond recognition by Kaiju, Transformers and colorful ponies. But, once again, they were reassured by the fact that these Kaiju cards were simply alternate-art treatments of other cards from the set. If you didn't want them, you didn't have to buy them. This is not true for Secret Lair X The Walking Dead.
In contrast with every Secret Lair released before it, Secret Lair X The Walking Dead contains mechanically unique cards legal in Eternal formats like Commander, Vintage and Legacy. Mechanically unique cards are nothing new, but Secret Lairs are sold at a premium. A Commander player who wants to have Negan or Michonne helming their deck will have to pay $50 for the entire set. This is $10 more than the price of the annual Commander preconstructed decks, which each contain dozens of new cards and valuable reprints.
While the cards previewed so far don't look too powerful, it's hard to predict what broken combo or interaction might gain prevalence in older formats like Legacy or Vintage. Players might end up paying a far steeper price for these cards in years to come, as Secret Lairs (while print-to-demand) are only available for a short period of time. Wizards of the Coast has already tried to assuage player fears about this, tweeting that these cards could see non-Walking Dead versions printed in the future.
While a non-branded version of these cards printed in a less exclusive set would help, it doesn't stop the precedent being established by this Secret Lair. Wizards can now print whatever card it wants to make sure a Secret Lair sells. There's already a bad precedent with mechanically unique cards being used as a promotion thanks to Nexus of Fate.
Starting with Dominaria, each Standard-legal set included a "buy-a-box" promo, which is a card outside of the main set only obtainable by buying a "box" of booster packs. Many of these fell into the same category of "designed with casual formats and Commander in mind." But Core Set 2019 saw the printing of Nexus of Fate, a card that spiked to nearly $30 and was banned from both the Arena Standard and Pioneer formats. A card similar to Nexus would rocket to even more absurd prices if it was printed in a Secret Lair, and that would be generally detrimental to the health of Eternal formats.
The oddest part of all of this is Wizards already has a solution for printing new cards with alternate branding. The company released a special limited card set with My Little Pony branding to raise money for charity, and it gave the cards a silver border treatment to prevent them from being played in tournaments. The Walking Dead Secret Lair could have easily received the same, letting Wizards create new card designs for the crossover without diluting Magic's brand. However, it chose not to do this, instead opting to make more money by cashing in on the health of older formats.