Beyond the issue of judicial activism, the idea of a strong legal obligation towards people who don’t exist is suspect.
One of the cornerstones of modern ethics is autonomy - roughly “don’t make moral decisions for other people.”
But creating a strong political obligation towards non existent people frankly violates their autonomy.
Who knows what future generations will want?
For example, perhaps these future people are strongly religious, and object to birth control. Perhaps they are radical right wing climate deniers.
Maybe they are literal Nazis who successfully wiped out everyone who isn’t of a specific racial background.
It might sound sad that future people would be that way, but remember they are all the children of whoever survived and reproduced.
Even more problematic: we are creating the very people we have an oligarion toward by our actions.
We could easily invent an international immigration policy that would create the people (and obligations) we want.
For example, a EU-rophile could say: “loosened immigration rules have created future generations with a more European wide outlook. Therefore they will need stronger transnational institutions to support them.” Or we just easily say, “People if the future are more likely to want the strong national identity we currently lack, so let’s focus on national policies.”