I'd rather have it empty and have to make up the game than risk the player pinch hitting or coming in as a defensive replacement. Late in the season when you're hopelessly behind the 162 games played pace you could turn the setting off.
Alternatively, or additionally, there could be a setting that swaps in a player with eligibility at the position with a green ^ symbol. I guess setting lineups could be considered a fantasy "skill" but the 4:30PM-6:30PM window when lineup decisions are being made by teams is not a great time for a lot of us to be hassling with lineups. Maybe this feature could be a setting that the commisioner activates or not.
I've been in leagues where busy grown-ups essentially could not compete because they can't check lineups repeatedly during the evening commute/dinner hour. Over the season you just fall farther and farther behind the games played pace.