It's a miracle that Miller Park got built at all. A lot of other ideas got thrown out there, but getting the ballpark at all, in the same location with the same set of parking lots, was a miracle.
To get the entertainment district off the ground, someone needs to throw at least a couple hundred million at it, it's not going to happen organically. A new parking structure to replace the lost surface parking would probably run $50-100 million by itself. It's not going to happen. Running public transit to the current site would be even more expensive and wouldn't make sense unless Milwaukee is planning to build a light rail network, which is definitely not happening anytime soon. The Brewers have clearly done the math and decided that it's not only economically unfeasible for them, but that it's also politically unfeasible to even ask for pubic investment to get it off the ground. I also agree that the Brewers will want to move in the middle of the century and they don't want to be stuck in the valley with whatever stuff gets built around the stadium. I think they will want a downtown Milwaukee ballpark at that time. No other location makes more sense than their current one.
Target Field is an interesting comparison. On one side is downtown, the basketball arena, and a vibrant entertainment district. On the other side is basically a bunch of highways and a dead zone (although there are some cool breweries along otherwise-deserted streets and they are currently expanding light rail in that direction). The closest Milwaukee equivalent would be to put it north of Fiserv on the old Park East parcels.
I know it's too soon to talk about demolishing the current stadium, but it feels like we're going to be spending a ton of money to delay the inevitable. I wish they could find a way to spend like $100-200 million to get the clamshell through 2035 and spend the rest on a downtown stadium.