Let's take my favorite anime for a moment. Magical Girl Lyrical Nanoha. If I were to take this question at face value, I think the franchise would completely die, and much of the fan base would be very upset if not outright angry. That could impact the creator's future projects in a negative way.
However, let's look at the franchise history of Nanoha, and in a way, the main protagonist did kind of die. She was sidelined. When the sequel came out (A's), it was mostly about Nanoha and Fate forming their friendship, and a new set of characters were introduced, including a third protagonist.
Then StrikerS came out. Nanoha is now grown up, and while she does have a role, it's more of a secondary role, not a primary one that a protagonist should be awarded. It's almost an offense to the definition of protagonist! Then other series come out (Vivid, and a few others?) -- yes, I'm stuck in like 2010, right?!) The star of the franchise becomes Nanoha's [redacted], and from what I understand (I haven't made time to watch it yet), Nanoha has even a smaller role.
To me, this is almost like the main protagonist dying. The fans, myself included DO want to see more Nanoha and Fate, I specifically would love to see more storyline between A's and StrikerS, but that's not the direction the creator took. And because of the path the franchise is on, there's like a hundred characters to keep track of. And each character is beautiful because there's so many layers to each one. Seemingly nobody is just a quick insert -> garbage bin plot device. If anything, I hope to achieve that level of character creation within my own stories. But I digress...