I've heard it strongly implied from several second-hand sources that if Zelazny had updated his will and placed the estate in Devin's name, then the prequels would never have been authorised.
Let me put it this way... I have zero interest in the prequel novels. Ultimately, they don't exist as far as my life goes. I will never have to read them, and I'll never use anything from them in an ADRPG game, whether as a GM, a player, or a developer should the pen-and-paper game make a comeback with any involvement from me.
I think Roger had it in his power to make clear his wishes about sequels in a legal sense, but he didn't. I don't really fault his wife for authorizing them, but I certainly wouldn't have written them, knowing his wishes on the matter.
Since I haven't read them, I can't be the judge, but I'm having a hard time imagining that they could be shittier than the visual guide, the choose-your-own-adventure books, or the comic adaptation, and they are likely to be far better than any Sci-Fi Network adaptation of the series was liable to be.
And sure, the kids were old enough to take care of themselves. That doesn't need they may not have needed money. I have no idea what financial state Roger's ex-wife was in... but I do know enough about how much authors make to imagine that she likely wasn't rolling in dough after decades of marriage.