By 2045 will baseball even be a major sport? The way it has been trending that would be a no. It is currently falling farther and farther behind the NBA and NFL. MLB is clearly the #3 sport in the US and it is far behind the NBA at #2 and the NFL at #1. Only the NBA has been gaining ground on the NFL in terms of popularity. It is possible soccer or another sport surpasses MLB at the #3 spot.
I don't think the money will be there for teams to move like they do now. The NFL and NBA sure but not MLB. There are only a few cities left where it makes sense to move to and there will be two less in the next 2-5 years. With the Rays getting their stadium situation handled and the A's looking like they will leave Oakland for Las Vegas it is only a matter of time before expansion starts. The MLB commissioner has gone on record stating once those two stadium situations are resolved the league will be expanding.
The Brewers will obviously ask for a new stadium come 2045 and they probably will need one at that time. I am just not sure the economic climate will be the same as it is today when it comes to cities bidding for teams. Nashville, Charlotte, Salt Lake City, Austin, Montreal and Portland are all possibilities for expansion in the next 2-5 years. Nashville is probably the only guarantee here to get a team as they have the backing already set in place. Will Nashville do two new stadiums in a few years? I am not sure on that but it is possible.
If the Brewers do ask for a new stadium I think they will move the stadium to a more financially viable site. Probably just tear down that whole area and create a business or industrial park. I am not sure where else the Brewers could move to once the lease is up but I don't think they would stay in the current area. Maybe they move to Mequon or the Brown Deer area.