It MIGHT slow down the format.
But do you want a slower format where your deck is either running garb, playing to beat garb, or both, with no other decks being viable, or do you want a format where the cards that push the game to be faster don't really exist?
The reason the format is so fast is because TCPI wanted the format to be fast, so they made a ton of super good basic pokemon with good cheap attacks. With sun and moon, they've shown that they're trying to make the best cards slower, so that the format as a whole will have more of a focus on slower decks.
But trying to instantly force the meta to be slower by warping the entire game around whether you can beat a single card is a patchwork solution at best, and not a good one.
As they release more good stage 1 and 2 GX's in the coming sets, and as the XY era starts to rotate out, the game will slow down dramatically because all the best cards are going to require focusing on slower strategies, but there's no need to try and force the game to be slower right this moment.