I don't think future expansion will have any harm on affiliate memberships for JHU and UND. First, hockey and lacrosse are niche sports. If you look at all the schools the B1G could possibly add, very few offer either sport and even fewer offer both. Second, lacrosse and hockey have the fewest number of participating schools in the B1G. Let's say a current B1G school adds hockey or lacrosse in the coming decade. Well, hockey only has 7 (with Notre Dame) and lacrosse only has 6 (7 for women with the JHU addition). Those are still fairly small numbers. It would be easy to expand and still play a round robin schedule. And if the sports ever grew so much in the conference that it would be impossible to play a round robin schedule, the B1G already has some Olympic sports where teams go a full season without playing each other (ex. baseball, softball, women's soccer). Maybe you just get to the point where you don't play a full round robin schedule in lacrosse or instead of a home series and an away series in hockey, you only play a team home OR away. I don't think those sports will ever grow to that point, but that is the easy solution if they do. I honestly don't see any B1G schools adding lacrosse or hockey anytime in the foreseeable future. And most of the realistic expansion targets for the B1G outside of Notre Dame or UConn would all be schools in a region where hockey isn't played and outside of the 4 ACC schools, a region where lacrosse isn't played. I don't see Kansas, Oklahoma or Texas playing hockey or lacrosse anytime soon. Same goes for Georgia Tech or any southern school outside of Virginia or North Carolina.
Finally, its important to realize that no one has ever left the B1G once a member of the conference except for Chicago who left D1 sports all together. Once you are in, you are in.