It gets discussed in Elogium.
CHAKOTAY: If it does take seventy five years, we're going to need a replacement crew in about half that time.
JANEWAY: Who'd have thought we'd be considering a generational ship when we were ordered on a three week mission.
CHAKOTAY: I know, but it's a problem we have to face now.
JANEWAY: What would that mean for the children? What kind of life would we be giving them aboard a starship, travelling through a potentially hostile part of space? And are we equipped to provide for their needs? Child care, educational facilities. We'd be building an entire community on board this ship. That's a massive commitment.
CHAKOTAY: Are you prepared to tell them they can't have children?
JANEWAY: I can't do that. And I've made it clear to Kes that it's her choice whether to have a child or not, but. There aren't any easy answers here. For any of us.
I think though that they've seen enough magic fixes, wormholes, the other Caretaker that they are still hoping for some kind of breakthrough. Once they established some communication with the Alpha quadrant that would have kept that hope alive as well. When did the E2 crew in ENT decide they needed to be a generational ship? Not in the first few years I'm sure.
I'd love to see a series that starts with the first generation born on a Generational ship as adults, the original crew still there but older etc.. trying to hold together the purpose of the mission. Let's say in 12 years VOY did decide to start breeding for replacement, You're going to have to indoctrinate the next generation into returning to earth being their heart's desire when they have minimal connection to it. Add a few more crew from the DQ and at some point you may have a rebellion wanting to settle somewhere.