[2021] How is Hullbreaker a replacement for Sanguine Blade?

Not entirely wrong, though I think the phrasing here leaves some things that can be misconstrued. When we design things we always have a specific goal in mind. I can't speak specifically to Sanguine/Hullbreaker, but for example when we make a champion we have a goal lane that we want them to be good in, and maybe another lane that we are ok with them also being good in. We actively want that specific champion to be good somewhere, so it makes sense for us to make changes to make them better in that role (and sometimes, worse in other roles). We can make the same correlation for items. For example its pretty clear we want Luden's to be a mage item, so if it was good on Tanks (somehow) it would make sense for us to make changes to make it either worse on tanks, or better on mages to try to reach those goals. Things get much more complicated when you really dive into the nitty-gritty, but that is a very simplified explanation.

Yeah we allow for some level of flexibility for sure, as long as its not game-breaking or unhealthy. Something like Viego mid or top being comprable to jungle isn't that unhealthy, but obviously if something like Yuumi jungle was good there would be clear gameplay problems.

We definitely allow for some level of flexbility, where they become a problem is where they harm the overall health of the game and create problems. Ziggs and some other mages being playable bot occasionally? probably not a big deal. Mages being so good that they completely replace marksman bot lane? Definitely a problem because its invalidating the marksman class.

I definitely find myself yearning for the days of old sometimes, I used to love playing Sona mid for example. I think a lot of the problem here stems from how hyper-optimized the game is now. Its great that everyone has reliable, easy access to really accurate data sources for just about everything for league, but one of the downsides is that often when something "unexpected" becomes even remotely strong, it completely takes over the meta. Good recent examples of this are Soraka/Sona top. Plenty of really strong, quirky stuff still exists out there I assure you, but the problem usually comes when it becomes optimal because then it tends to start causing problems.

Depends on the champ. As long as they aren't overpowered or dominant for the most part they aren't that bad. Most only exist as niche counterpicks anyway, which will always exist. Something like Kalista might be a problem because Kalista in a solo lane does not utilize a large portion of her kit, for example her passive on W and for the most part her R. If a champion is functioning without some of their abilities that is likely not something we want, so we might look to make changes to help rectify that.

It happens, champions migrate lanes. Nocturne for example used to be almost only a jungler but now he is very much in solo lanes. We hold more stricter goals for new champions as well as champions we are actively making changes to, but it's simply impossible to enforce strict goals for every existing champion in the game. If we deemed that Lucian mid/top was unhealthy for a specific reason or perhaps it became a pro play problem, then that might lead us to make some changes.