I wonder, what theoretical features could we have for a modular grid that'd be fundamentally incompatible with a columnar grid, and vice versa?

If adding columnar grids to CSS Grids doesn't make any features impossible, then I don't see why we shouldn't just have one CSS Grid standard, and thus always ensure feature parity between modular and columnar grids.

The argument being portrayed by Simmons, as I understand it, is that every feature that currently works for CSS Grid's modular grids, would also work for a columnar grid.