Just a holdover from the early days of the game. Plus the buildings in a specific chapter were really only good for that chapter and maybe the next. You are supposed to want to get rid of the old buildings and decorate with the ones from your current chapter, not keep that chapter 1 culture building when you are in chapter 5. And the premium culture buildings was the way Inno made money in the early days of the game, when there was a lot less to spend diamonds on. I started playing almost four years ago and I remember having quite a bit of the normal culture buildings in my city because I needed the culture and there was almost nothing else available. I did not earn a single event building in that city til I was somewhere in chapter 3.
On the premium buildings specifically, I think they should be eligible for royal restoration upgrades. We spent money on them, so we should be able to keep them relevant. But not the normal buildings. They are not worth upgrading.
And yeah, all the events, and the buildings from them, have really killed any need to buy any of the normal buildings, unless you really like the look of something and don't care that it is inferior in stats. For example, in my new Beta city, I am midway through chapter 3 and the only normal culture buildings I bought were ones required by quests, and once the quest was complete, I sold it off again because that space was more valuable for placing the much better event buildings I had already won in the Air Traders event there while my city was still in chapters 1 and 2.
TLDR all the culture buildings you can buy are really good for nothing more than looks now. If people want to give their city some variety and make it look interesting, they will buy those buildings. If not, they will ignore them, like I do in my Beta city.