This decision was decided so mutually that I don't even know who mutually decided it first.
I've been a writer for a long long time, and during that long long time I've figured out what I can do and what I can't. I can do rewrites. I may gripe and complain about the process, but I've worked with quite a number of excellent editors who've helped guide me as I've tried to make my book better.
What I can't do is write to please someone else. It's simply not a skill I have. Even when I've written books pretty much just for the money, they've been stories I've enjoyed figuring out, with characters I've wanted to get to know better. For better or worse (and it can go either way), I write for myself.
Emily Bronte wrote a poem that has a couple of lines I've always loved.
I'll walk where my own nature would be leading:
It vexes me to choose another guide.
I could devote the next few months of my life to trying to figure out an approach for a fourth moon book that would please my publisher, but frankly, I avoid being vexed. Therefore yesterday afternoon I dug through my file cabinet, found the unsigned contracts for the fourth moon book, and mailed them back to my agent. This wasn't a foot stomping whining tantrumy decision. It was simply an awareness that a fourth book wasn't going to work.
So if you want to know what happens to Miranda and Alex next, you'll have to decide for yourself. Maybe they'll be happier in your mind's eye. Maybe they'll be sadder. But either way,you'll take them where your nature would be leading. And that's just where they should go.