Yeah, but it's super abstract. By the time we realized that Iorveth's quest wasn't working (he was actually a secondary character in a much bigger story about the war) it was way too late to add something new. About the only thing we'd have had time to do would have been to put his dead body somewhere with a note, which was suggested. I think that would have been a pretty poor conclusion to his story, so it was better to just leave him unresolved.
For an idea of what we need to do to even add a small quest. First I need to draft the quest in paper, get approval from quest lead and the director. This can actually take a couple of weeks, longer if the quest is important. Level design, environment art, character art, audio, cinematics, animation all need to get asset requests. For a small quest I'd try to avoid making more than one new character request, more than 2-3 locations. Some of the assets will go to outsource, most will be done in house.
While that's taking place (months and months of work) the quest and story teams draft the quest. This can take a while, since a lot of stuff that works on paper doesn't work in game, for many many possible reasons.
Once story is done, the text goes to localization (our writers all write in PL). From there lines are translated into a dozen languages, then sent to the studio to get actors to perform them. Those lines flow back to audio and cinematics, who put them into the game and make sure they sound right.
Meanwhile QA and quest bug-fix and iterate on the quest, basically until we ship. This includes bugs of course, but also general feedback. During this phase entire locations might be moved or redone, characters change, etc, and each team needs to respond to each change. So if QA realizes that we made a mistake and *actually* this character shouldn't say this or this... also he should have a red sash instead of a blue one... also his house is in a swamp and it needs to be in the forest... etc. Each of those changes means that different teams need to jump in. Textual changes are particularly nasty because it means literally hundreds of people, producers, translators, managers, actors, directors, audio technicians, cinematic artists, etc, all have to deal with it.
So I mean, in the super abstract, yes, you are right. Giving Iorveth his own side quest wouldn't have been hard. Actually, we gave him a huge quest and multiple side quests and it was even playable*. But it got cut and it wouldn't have been simple to replace it at that stage of the project.
*Asterix to remind you that as a dev, my definition of playable is not the same as yours. No one who hasn't worked in gamedev really has any idea how janky and ugly games are until very very late in development.
The quest was pretty nilfgaard-centric and involved a renegade general (and demonologist) who was doing crazy wizard poo poo in Velen. Thaler was there, and so was Vincent Meis (but he got cut early). Iorveth was in there, trying to steal something from the demon-summoning general guy so that he could cure a plague that was killing his doods. Some parts of the old story stuck around, Eye for an Eye, Patrol Gone Missing, and... uh whatever the one is with the lady who wants you to find her redheaded kid? There was also a tiny quest in the Nilfgaard camp about some soldiers who stole a pig and tried to make it look like a monster took it. It got cut because it was bad (It was my quest, I'm allowed to say that.)
The whole nilfgaardian war stuff got cut for mostly just not fitting in well with the rest of the game and simply not having enough time to finish it properly.