Your argument makes a major assumption: That this isn't already the case.
The current system is regressive in practice, though not in theory. If this wasn't the case, Buffet wouldn't be paying 17%.
Spending 100%+ of your income on consumption can be the norm for lower income earners, it is true. That does not extend however into 100%+ of income spent on consumption that is required for basic sustainability, especially in rich countries such as in North America or Europe.
There is no incentive under the current system for any income bracket to spend within their own means, regardless of what those means may be. The high levels of unsustainable personal debt illustrate this perfectly.
This results in overspending on "luxury" (i.e. not "required") goods - even by the "poor". There are few incentives that encourage the poor to establish wealth through savings. There is also no incentive in the system to have individuals increase their income to match their spending, although that's admittedly a much harder accomplishment.
With an income tax system that isn't perfectly flat, the problems you highlight are simply hidden, if not encouraged. With a flat based taxed, they are not, but it is regressive.
With a flat consumption based tax with a level of social control through either personal spending exemptions or through item based exemptions (baby food, for example) we could do a better job of managing the regressive nature of tax systems in general than we do right now.
In addition, we expose basic economics to those who have difficulty understanding the concept by providing strong incentives to build wealth and consume less, regardless of income bracket. This is something we should do.