Unless it's for the absolute worst circumstance, like it was with
Lexi, I don't understand how a game would end up abandoned. Are there really people out there who will start a project with no end goal? People are really writing these stories with no conclusion? (Rhetorical questions. Obviously people do, based on the amount of abandoned games on this site.)
I'd say very few amateur indie VN devs start a game having a full idea in mind of where it's going. Many of them just start with a cool idea and assume they'll wing it as they go, not realizing just how painful that sort of development style can become as you get deeper into things. Even devs who know where/how they want a story to end don't necessarily plan out the path to get there in advance, and instead just sort of improvise the smaller story beats within the context of the larger arc.
It's probably really rare that a dev actually sits down and writes the entire script for a game out before they start to code.
Then you've got the problem of catering to Patreons or other supporters -
you might have planned for that one character you introduced to be little more than a throwaway NPC in a single scene, but if tons of the people throwing money at you to make the game seem to love them and keep demanding more scenes, what do you do? Stick to your original vision and potentially lose money? Or rework the plot to give them a larger role, potentially causing other problems? It's easier to improv on the fly if you never really had an end-game in mind in the first place, but you can still make things confusing for yourself, or at worst even start to lose passion for the project because now you're incorporating characters or fetishes or other ideas the supporters want that you personally may not like.
Honestly, I think most abandoned games wind up that way because the dev starts out working on something they consider a passion project based on an idea or three that excites them, only to become incredibly overwhelmed when they realize just how much work it is (especially as the game grows bigger and coding gets more complicated). And that's not even getting into games where the dev is initially okay with the amount of effort, but then something happens in their life (relationship problems, job issues, health scare, etc) that suddenly make it much harder to put the same amount of effort in. So what once seemed like an accomplishable goal starts to look like something they'll never be able to finish, so they kind of just drop it out of helplessness and frustration.
I may be sad when a game that seems awesome gets dropped in the middle of development because the dev wound up overwhelmed by the whole mess, because they're only human, and for most of them, this isn't even their actual job. I think it only really becomes a problem when it starts to feel like a dev is deliberately stringing supporters along because they want to make tons of money off a project they have no intention of ever finishing.