With the recent increase of multiple-wide receiver sets in the NFL, many analysts have declared the nickel defense the new base defense.
Base-defense usage is surely decreasing, but expect the San Francisco 49ers to line up in their new 4-3 defense — similar to the 4-3 under/over-hybrid defenses, which defensive coordinator Robert Saleh has worked with in the past — more often than the average NFL team.
When Saleh worked for the Seattle Seahawks and Jacksonville Jaguars, the teams often kept their base defense on the field against three-wide receiver sets on early downs.