I think the "I Did It" t-shirt for purchase (not as the included race shirt) is traditional, they've had them about as long as I've been doing Disney races (before runDisney was runDisney). I like that they continue that tradition.
Long ago they'd sell a Champion-brand "I Did It" t-shirt featuring a cool graphic of the race course map on the back of the shirt. Those were my favorite.
I don't like "I Did It" on the medals and mileage magnets, though! And it certainly doesn't need to be the most prominent feature of the finisher tshirt.
Unrelated, but I am VERY HAPPY to see they brought back some raglan-cut sleeves on the I Did It shirts, now they need to do the same on the official race shirts!! Raglan cut is better for running/ease-of-movement and often fits better for women and people with athletic builds (like runners!). Disney cheaped out big time when they changed their shirt cuts several years back so this is a change back for the better that I hope is expanded. (I rarely wear my new, cheaper runDisney shirts but my old raglan-cut Champions still get heavy use!)