I think you misunderstand. Increasing payroll does not automatically confer a more effective team. Increasing scouting budget does automatically confer more effective scouting. It is unrealistic to go from having the best medical staff in the league to the worst, and vice versa.
"Budget" does not mean the same thing for payroll as it does for scouting. Thus, there should be a different rule.
If scouting could be so easily flip-flopped, I would simply alternate between 20 and 0 and just draft unsignable players when I had 0. I don't think there is a similar abuse that can be made through player payroll. Although, in all honestly, I wouldn't mind seeing a limit (10M?) to how much player payroll can change from year to year, because you rarely see such a drastic change in a team's payroll as you can see in HBD. I think it could also add an element of strategy by forcing players to plan ahead. But I think prospect payroll should be allowed to change freely. Teams often go from being relative non-players in the amateur market (in years where they are spending more in FA) to being major factors, and vice versa. I know the Phillies did that recently.