Well that would benefit the rich more than the poor.
Currently, the rich are benefiting from the fact that capital gains are taxed lower than income (unless they're short term). Abolishing the income tax but keeping the capital tax would unironically lead to the precise outcome that leftists always say that they want — namely, that the rich pay more taxes than the poor.
It would also encourage working rather than owning or investing, and it would probably even make domestic labor cheap enough to be competitive with illegals. It's unironically the best policy Trump could possibly enact, because it would encourage more American citizens to work and in turn reduce our dependence on brownoid slave labor, which means more of them can be deported.
I sincerely hope he gets this done. It would absolutely change this country for the better.