One of the trends that have led up to these salaries is the concept of "franchise quarterback". The franchises make a financial commitment that requires the QB to start and so all competition is cut off.
The GM's don't dare acquire a backup with the potential to be better and replace their multi-million dollar investment. There have even been star QB's that were upset about the possibility of potential competition.
As such, there is rarely a backup QB that doesn't represent a serious reduction of productivity. The agents for starting QB's must love this, they can ask for the moon because if that QB sits out then that team's season is probably over before it started.
This would never happen in the era before free agency. Teams were built on a career commitment by each player since they would have to be traded or cut to play elsewhere.
Backup QB's never had the option of going somewhere else after 3-5 seasons. Some teams had up to 3 QB's that could legitimately start.
Steve Spurrier, a Heisman trophy winner and 1967 3rd overall draft pick of the 49ers sat for 9 seasons behind John Brodie before being a full-time starter.
Today, many new QB's get almost zero time to acclimate to the pro game, there is very little development. As a result, the list of viable options for a team that covets a good starting QB immediately is practically non-existent.