The Interim Champion should become the Undisputed Champion once he has successfully defended his Interim title as many times as the Undisputed Champion has defended his, plus one more title defense.
For example, Dominick Cruz has defended his title 2 times, therefore Barao should become the Undisputed Champion after 3 title defenses. By surpassing the Undisputed Champion's amount of title defenses, the Interim Champion has accomplished more as Champion than the current Undisputed Champion, therefore he deserves to become Undisputed.
A fighter like GSP who has defended his title an astonishing 8 consecutive times shouldn't be dropped as Undisputed Champion after a certain time has passed by. His accomplishments inside of the octagon should be recognized, therefore if there were an Interim Champion in the Welterweight Divison and GSP was injured, the Interim Champion should have to defend his belt 9 consecutive times in order to become Undisputed.
If you can't fight the Champion, you have to beat his record to take his belt.