DL has done just fine for years with Gadget's Go Coaster (the same thing basically as Barnstormer) together with the Matterhorn (which was for a long time a 38" "starter" coaster that kids could enjoy). Don't see why this would be any different with MK.
Because it's a different environment.
Factor in a few things.
1) Families who travel to Disney World (on average) are there on vacation, as opposed to going in for the day.
2) Because of (1), families only have so much time to hit the top attractions during their stay.
3) 7DMT is a buffed-up, expanded, and better themed Barnstormer ride (that's not an insult, that's just reality)
7DMT will look "cooler" to little kids when compared to Barnstormer. Therefore, combining all of the factors above, what purpose does Barnstormer serve? It pales in comparison to 7DMT. It's like having the Haunted Mansion, then down the road have a carnival Haunted House ride.
DL gets away with it because of the plethora of dark rides (and rides in general) within their Magic Kingdom, in addition to the laid back formalities of most of their guests. Also, the lines are not as long. I wouldn't mind riding Gadget's Go coaster if the line is 5 mins, regardless of whether or not Matterhorn is next door. Something like a 60 minute wait at Disneyland is considered long and rare unless during peak times. At WDW for some attractions, especially Fantasyland ones, 60 mins is an average wait time.
Kids are going to want to ride 7DMT over Barnstormer as their "starter" coaster on their vacation. Expect those families to either have a FP+ for it, or wait a very long time. After waiting a long time, why would they use more vacation time to go ride Barnstormer, if they basically just rode a better version of it?