I'm personally not a fan of series,, in general, but some love certain series (LOTR, etc)
I'm not saying that a series necessarily makes bad books. And if you like series don't mind me. What do I know other than my unjustified and unjustifiable opinions?
But what I think I know is that the beady-eyed gnomes in the publishing office use the series concept to lock an author into multi-book contracts when the 3 strawberries line up in their minds portending big sales. So they not only grow fat on the first book but also the rest of the books in the contract. It's very much the same for production companies and producers to declare a movie a franchise. These are sales tools of the greedy bastards, and they are very effective not only on the consumers of the art but also those who produce it.
Now the writer can also make out by writing a series but only if he doesn't give away the farm at the first taste of real money he has likely ever had. I would say that if the publisher attaches a provision that they acquire all rights, be willing to walk away. And I would want to be dead certain that I could live with writing the same book over and over for 5-10 years.
I've had an article on the back burner that would be a fusion of Mahayana, Yogacara and the only true philosophical examination that I know of Buddhism by Dharmakirti and Dignaga. But the publisher thinks that he can walk away with the rights. Quite likely he will never publish it because I would rather let it rot on my disk than agree to his terms.
Such publishers set the condition that to be published by them means you surrender all rights. I wonder if they care that a finished piece means being paid no more than $5.00 an hour. The best writing has seen draft after draft after draft. And you want to to waltz away with all rights to my work?
You are not getting near it.