Marketback Walker is good at different points in the curve: If you cast it for 4 mana, you can have a 2/2 that dies into two cards, and so on
Seven cards all the time
It's "just" a one-for-one, but if you either drew more Lands than your opponent or they just had a light draw, it could result in some un-fun non-games, especially in an era with less friendly mulligan rules
Now, if you've missed the other posts in the Young Money interview series, I highly recommend going back and giving them a read
(2021-03-19) If an effect uses the word "search" once but allows an opponent to search for multiple cards, the top four cards remain the same throughout the entire search
Also, to make it less of a feels-bad, he has a way to remove the maximum hand size limit, baked in