The thing we must remember about the open endedness of TWOK was that it was indeed motivated by more than just money.
At the time, Leonard Nimoy was adamant that TWOK would be the LAST Trek movie he would do. (Of course, I think in later years, we've heard that same claim). When he was offered the director's seat for a sequel, he relented.
Also, the initial test screening with a more somber, more "final" ending did not meet with much approval from test audiences...so the ending was reshot with a bit more hope.
But, I guess, in the end, rectifying all of the above does have much to do with money.