"What people would pay for it" is a controversial quantity that can only be speculatively and uncertainly answered unless you are actually selling it in an open, unrestricted auction (even actual sale under real-world conditions often doesn't answer this, since its possible that both buyer and seller have non-financial interests which motivate the sale decision such that the purchase price does not represent the market value.) For pure interchangeable commodities with robust markets, this is less of a problem, but for many real world assets (including real estate) this is not the case.
Note that while real estate is already subject to ad valorem taxes (and other value-based policies, such as eminent domain), valuation is not at all free from contention even with those taxes as just one piece of the overall tax picture; if wealth-based taxes were the primary tax system, that problem would be magnified.
> How do you determine income objectively?
If you can value wealth objectively, you can value income objectively, since income is just a subset of the changes in wealth. The problems in income valuation are problems in wealth valuation (which is why, e.g., capital assets are valued at sale for income purposes, which becomes less viable of a solution to the problem of valuation if you aren't taxing the change in wealth but taxing the wealth in each year it was held.)