Re https://www.w3.org/TR/css-grid-1/#overlarge-grids
"Clamping Overly Large Grids"
"For example, if a UA only supported grids with at most 1000 tracks in each dimension"

Some might use CSS Grid for eg spreadsheet-like web-aps, or for crazy layouts where each cell is like a dot in a dot matrix, and there probably are more use-cases for high track counts. These Grid users should be able to rely on some level of track count. Perhaps have a minimal required track count / column/row count?