Yeah I understand what you are describing.
The reason the site does not persist lineups is because it would break the weekly lineup view. I highly recommend everyone uses this view. If every time you put someone into a slot for a specific day, it then set that player in the slot for the rest of the week, it would make setting lineups a really bad slog and undo most of the value of this page.
On the flip side, people not setting their lineups is definitely a competitive problem in all fantasy sports. Generally I think the right decision is to replace managers who aren’t setting their lineups, but I also understand there’s a good argument for a default lineup to start each week just so teams don’t stack up 0s.
I’ll think more about how this can be accomplished without ruining the process of setting lineups for teams that are active.