The buy/borrow loophole is pretty much built into any tax based on income (due to the realization requirement on cap gains and the fact that borrowed money is not considered income). Good luck on closing it without changing the tax base from income to something else. And a wealth tax would probably be counterproductive as rich people would flee, except for taxing property which people can't take with them.
Besides, I already pointed out in my original comment how a consumption tax can be just as progressive as an income tax. Why do you keep insisting that it has to be regressive? Is it because the tax isn't imposed preemptively, before people decide to spend? People aren't going to hoard their money forever - eventually the heirs will squander it, as happens to all fortunes eventually, and then it will get taxed at a high rate (and the people squandering it don't care about the tax, which makes collecting tax a lot easier than if it was imposed on the people originally earning the money).
And in the meantime the payroll tax, which is literally a regressive tax, and is a bigger burden to low- and middle-income people than the income tax, continues and no one cares about it. If you're concerned with loopholes, start with that one.