Well, not if you need database primary keys with sensible index locality. Which is what this draft is about.

Aren't indexes sorted? I suppose using time as the MSBs means new batches are always appended (after sorting just the new ones).

Yes they are. But, consider the common query, for records based on an time index on the last day. This index will point to a bunch of primary keys and those, being random, will be spread across a much wider range of blocks. If the primary key was just the time, those items would be on the same or close blocks.