Flat tax is not just about removing thresholds, but also using the same tax rate across the board. If you tax in the same way income, capital gains or corporate profits, don't give a tax exemption for debt, don't charge a stamp duty on property (or charge the same rate on the purchase of all assets, property or not), etc, then you kill all these tax schemes.
Isn't that usually called a broad based tax or so? I mean I mostly agree with what you are saying, just noting that I'm used to different terminology.
Yes, it's a good idea to put eg debt and equity on the same level; don't tax transactions that improve economic efficiency (tax consumption instead for example).
Land value tax is still a great idea for these and other reasons. Eg you can jack the lvt really high and basically tax away all land rent without negatively impacting the economy. So even in an otherwise 'flat tax world' like you describe, it might be a good idea to get the majority of revenue from taxing land rent (and in the wider sense, resource extraction and commons like pollution carrying capacity).
LVT sounds good in theory, but a large number of rich people _are_ acting as rent seekers (they own property, and extract value via rents on people who work/produce actual value on it). It would not be politically feasible, unless the gov't is prepared to fight against those powerful land owners.
Yes, the political feasibility is one big problem. (The other challenge is to figure out how much revenue you could raise---there are some good theoretical arguments that elimination a tax on something else like labour income should increase the local land value and thus land value tax by enough to offset that loss, but I don't know whether that works out in practice.)
There is some hope for the political feasibility: some countries already have land value taxes with various imperfections like Estonia, and some Australian states, and some counties in the US.
