Personally I think the moon was abandoned because they couldn't find any significant energy sources. No water to make fuel or power moon bases. In the past 30 years, we've found evidence of possible ice fields, learned a lot more about solar power, learned how to scrub co2
out of the air, learned how to get o2
from rocks. So the moon is looking a lot more possible now.
As for the comment about the 'Balance of the moon and the earth', the moon is at escape velocity and is going to leave orbit eventually. Weighing it down would actually help the situation most likely. Only problem is, taking mass from the earth to the moon doesn't affect the balance at all. Thus we can build anything we want up there, the only considerations are temperature and radiation.
What I think will become an issue down the road, is if we set up a moon base, they are going to want to use local materials to make fuel. I've only heard shakey evidence of ice in the bottom of a few craters. Wich means the amount of fuel available up there is going to be limited and eventually a source of conflict.
Still the moon is worthwhile simply because it's our first steps off this dying rock we call earth. We really don't have a choice about leaving, eventually the whole planet is gonna go splat. So it's best to run while the runnin's good.