Good points! But according to Quinn on Voyager, it seems that it's possible that the Q Continuum isn't as powerful as they pretend to be. A fixed amount of energy they have for their shenanigans (especially if they are based on technology) would fit with what he said.
Well, Q has existed for at least four billion years... I would think that once he reached, say, the 5% mark (200m years left), he might start living a quieter life, stop zapping things up randomly. But even in his last 30y of existence (proportionally, that's like about 25s for us), he was going full bore.
They really didn't explain it. Pretty terrible writing and another terrible end to a tng character. I fear the entire tng crew is in danger of rotten endings come season 3.