Doesn't really make sense. As
@Pape said, the teams themselves are making these deals - no one has forced them. So they want to protect themselves from themselves?
In general I think having a "max contract value" makes some level of sense but it shouldn't be tied to QBs only, it should be all players. And it should probably be a figure that adjusts every 3-year-window or something like that to keep up with the cap increasing. However, I think the other side of that coin means that more players (like star WRs, DTs/OLBs, etc) will demand higher deals because more cap money will be available since QBs will be taking up less. So at the end of the day will anything necessarily be gained?