Adjusting an age-ratings system doesn't take liberty away from anyone. Parents can still allow their kids to play whatever games the parent deems is ok.
I agree with some of your other points, though: we should have legally mandated return periods for this sort of thing. Not sure how you'd enshrine price limits into law, though; that seems impractical.
> Not sure how you'd enshrine price limits into law, though; that seems impractical.
Thinking in childrens' terms:
• Any microtransaction <$1 is fine, up to 10 per week or 20 per month or whatever
• Anything between $1-$10 should be more limited
• Anything $10 or above should be limited to 1 per week
• No microtransaction should cost more than 50% of the game's own full price, if the game isn't free