Woulda, coulda shoulda, but I imagine that if A. left rather than Marcy there was Robert Barr to take up the reins, except that the Dobsons didn't create that character so they were not motivated to write for him.
Going back to the money and recasts for a moment, a behind the scenes mismanagement story that I always recall about SB is that Terry Lester left because his agent called him to say that nobody in production had tried to renegotiate when his one year contract was set to expire. As a result, Terry just packed up on a Friday and never returned to set. One suspects that getting Gordan Thompson to start so quickly after Dynasty was cancelled cost a pretty penny. One can also suspect that Eileen Davidson (and later Kim Zimmer) didn't come cheap. So, despite the fact that they were loosing viewers 3:1 over GH, they were paying hefty fees to the actors with very little impact on the ratings.
Coupled with the reportedly large payout the Dobsons received from their lawsuit undoubtedly soured New World on wanting to continue spending money on a sinking ship. Especially when most of their other projects were outside of daytime.
She had a good contract with NBC for other primetime movies (according to IMDB after Palace Guard she starred in four NBC movies), so I think it was simply money. I recall when she was later hired at GL, there was press about how much she was paid, and one assumes that SB did not have the budget to make her an offer she couldn't refuse. Or, given all we now know about the mismanagement at SB, perhaps there wasn't enough money to make her want to go back.