What was your point then?
Laffer describes voluntary behavior in response to higher or lower costs. Your example family of 4 making $60k doesn't have the option to offshore their income to hide it from taxation. So why should we be looking at them?
You asked the question: Why aren't we? My answer is, because it doesn't apply.
After you show why Laffer is meaningful at the bottom end of the income scale, then you can show how much tax revenue we could expect to gain from changes there. Then you can compare that to the amount that could be gained from returning the top tax rates to historical averages.
If you can do all of that, then I'll believe you honestly think tax rates at the bottom end are a meaningful part of this discussion. I'm convinced they're not. And I'm pretty sure you know that too.
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Drew