TPTB wrote themselves into a corner, they had just reunited the show's golden couple, however, VP had given the top brass prior warning that she would be leaving, this was in 1985 and she left in 1987, they had two years to plan out her exit, what I don't think they counted on was Patrick Duffy returning after Bobby was killed off, which did throw a spanner in the works, my guess is that had Bobby stayed dead, Pam probably would've left Dallas with Christopher and carried on living off screen.
I guess they couldn't have Bobby or Pam cheat or meet someone else in order to break up the golden couple so maiming Pam in a massive car accident was their only way.
But given recent mishaps in the show, they couldn't recast the role immediately nor could they outright kill off Pam, TPTB were afraid of both options.
Perhaps they should've had Bobby cheat with Jenna and Pam finds out and she leaves Dallas, while Christopher makes occasional appearances, Pam however could leave Dallas pregnant with Bobby's baby.